ISentientSwordEffectProvider is an interface used to allow the off-hand item to provide bonus effects to the sentient sword (and also all of the sentient tools, but not the bow) when it hits a target. It has a basic (but turned off) implementation in the current Air Sigil code, but hasn't actually had any other effects coded, so I thought, why not put together that code.
I'm making this issue just so discussion about the effects, balance, and all that sort of thing can be put in one place and kept track of.

So, current ideas, and I'm allowing duplicates because there's only so many fitting things you can do with the current way the interface is implemented, basically just doing something either to the player or to a target on it getting hit:

• Air Sigil: Levitation of course. Current duration is 10 seconds of levitation 1, I believe. That might be a bit much though.
• Water Sigil: Drowning. Don't know how to make a target suddenly take drowning damage, but hey, there it is.
• Lava Sigil: Light the target on fire. Best implementation would probably be making the sword act as if it had 1 level higher of Fire Aspect than it already does, otherwise it would conflict with said enchantment.
• Void Sigil: A bit of extra void damage. Not a lot, but it ignores armor so that's pretty powerful.
• Sigil of the Green Grove: Poison
• Sigil of the Blood Lamp: Glowing
• Sigil of Elemental Affinity: Negative fire resistance, so extra fire damage. Lorce also suggested making targets take extra water-related damage, but I've no idea how to do that.
• Sigil of Magnetism: Negative jump boost. Pull them down to the ground. Extra falling damage and with high enough values can just plain stop things from jumping up blocks.
• Sigil of Suppression: No idea.
• Sigil of Haste: Slowness
• Sigil of the Fast Miner: Mining Fatigue
• Sigil of the Phantom Bridge: Nothing good here
• Sigil of Compression: No idea.
• Sigil of Ender Severance: Poisons endermen severely
• Teleposition Sigil: Random teleport
• Transposition Sigil: Random teleport, surprising I know
• Sigil of the Claw: Weakness
• Sigil of Elasticity: Increased knockback, perhaps?
• Sigil of Winter's Breath: Slowness

Possible addition:

• Make potion flasks implement the interface too and let those apply their effects to a target on hit. A use for all those negative potion effects, at least until we have throwable flasks.

Suggestion/enhancement.

Why not just make a sigil of holding in shield form? Could be a good way to both upgrade your sigil of holding and provide shield variety.

Hmm. The tricky question is what happens when the shield breaks.

Not really? There's plenty of ways to deal with that.
It probably wouldn't even have durability, and would use lp from network.
If it did you could have it either eject items or do like tinkers where it turns into a broken item that doesn't do anything other than let you access the sigils inside it.

First off, I would suggest putting the sigils in different tiers for complexity.
Basic sigils (air, water, lava, void) give basic (boring) effects that might be similar but each have their unique "element".

Elemental Sigils

For all the Elemental Sigils you could outright check if the target has a "Sigil of Elemental Affinity" active to prevent their effects from applying.

Air Sigil

Levitation is good, direct fall damage or setting a fall distance, really, would be my alternative suggestion to blend in with the other suggestions of mine. In either case it is countered by the "Sigil of Elemental Affinity" but levitation on its own might be a letdown as it might give your target an advantage or in case of a short duration won't do anything than impair movement a bit.

Water Sigil

Bad idea. There's probably no other option for "Sigil of the Fast Miner" and having two sigils with the same effect is bad design. I suggest extra drowning damage which should be able to be countered with the "Sigil of Elemental Affinity" (might not be because water breathing only prevents you from losing oxygen, in that case add immunity to drowning damage to the "Sigil of Elemental Affinity" or the water breathing effect in general).

Alternative suggestion:

• Applies effects that you get when you're under water:
  Reduced vision, losing oxygen, reduced movement/mining speed etc. Emulate water behavior for that so it can be alleviated with the corresponding potions/enchantments (depth strider, respiration, water breathing etc.) Effect name: "Submerged"

Lava Sigil

Same as above, there is already Fire Aspect. Higher levels of Fire Aspect only increase the duration which is redundant as you'll probably be attacking constantly.
I would suggest extra lava damage which is countered completely by the "Sigil of Elemental Affinity" or the respective levels of Resistance/Fire Resistance/Fire Protection/Protection and also reduced by armor. You could go as far as checking whether or not the target is affected by "Submerged" or under water to reduce/remove the damage in those cases.

Alternative Suggestion:

• Applies effects that you get when you're under lava:
  In addition to lava damage which would already be very strong, you would experience near blindness and be affected by burning (though I'm not sure how exactly the interaction is, it always seemed that you're only affected by the fire damage once you leave lava)

Void Sigil

As with Lava and Water, this one would deal void (out of world) damage. Bypassing armor and able to kill people in creative mode, this should deal less additional damage than both the "Water Sigil" and the "Lava Sigil". It should only be worth using if the enemy has a lot of armor. It also cannot be countered by the "Sigil of Elemental Affinity" as it is the absence of elements.

Alternative suggestion: Things you might connect with "void" or "nothingness":

• Blindness
• Invisibility (for the wearer, could try to not render stuff held in hand to make you invisible until you attack (which makes you visible for a bit))
• Spectator mode (something similar, really)

Second Tier Sigils

These Sigils would be slightly more powerful than Elemental Sigils or be more specialized.
They might afflict targets with effects that work against them or build on already existing effects, strengthening them. A second tier sigil might do nothing at all on its own.

Sigil of Elemental Affinity

As first example of "doing nothing on its own" and "increasing the effects of other sigils", the "Sigil of Elemental Affinity" might give the target an effect that I would call "Elemental Vulnerability", which increases fire/lava, drowning and fall damage as well as (if you decide for the Levitation effect on the "Air Sigil") increased levitation speed (so it will actually do something in a fight). If you want to be especially vile, it might also affect the target with any (or all) of the following alternative suggestions:

• Sets remaining oxygen to 0
• Removes effects that protect you from elemental damage (Fire Resistance, Water Breathing)
• Deactivate an activated "Sigil of Elemental Affinity" in the targets inventory

Sigil of the Blood Lamp

Glowing is the right direction but by itself a bit... weak.
I'd suggest adding the possibility to shoot a projectile on attack if you don't hit anything in melee. This projectile would deal a fraction of your normal attack as damage/fire damage and afflict the target with the "Glowing" status effect. It might also set the target on fire (the Blood Lamp projectile does that when you hit a creature with it).

Sigil of the Green Grove

Poison is good. Duration either adds up or resets, multiple attacks might have a chance to increment the amplifier level. This could also mean that it only has a chance to afflict the target with poison, as poison is fairly strong.
Keep in mind that there are mods that add a "Venom"-type enchantment for weapons that do basically the same so something else might be considered to remain halfway unique.

Alternative suggestions:

• Fake Sugar Cane blocks in a 3x2x3 centered on the target to reduce visibility
• Making the target stuck (rooted) on/in vegetation blocks (grass, vines, leaves etc)
• Guaranteed poison on/in vegetation blocks
• (a chance of) Life leech (careful with this one, it gets very strong very fast) (on/in vegetation blocks)

Sigil of the Claw

Either bleeding (damage over time, maybe based on current health) or things that I wrote under "Sigil of the Green Grove" (stuck because the actual use of the "Sigil of the Claw" is clinging yourself to a wall or consider yourself a predatory animal with claws which means "consuming" your foe (Life leech) or, as mentioned at first, bleeding.

Sigil of Winter's Breath

At first, I didn't consider it. Then I thought it should be added to the Elemental Sigils.
Then I thought: It's a bit more advanced than Elemental Sigils, second tier is better.

Everywhere: Minor slowness, minor increase/decrease in water/fire related damage, minor hunger/exhaustion.
Conditional effects if you're in a cold climate: Stronger slowness, major increase in water-related damage, major decrease in fire-related damage, major hunger/exhaustion.
If you're wet: Minor slipperiness.

Sigil of Elasticity

Very, very, very slippery. No fall damage. Bounce stronger than original fall.
(Ultimate troll edition)

Alternative suggestion:

• any of the above without a stronger bounce than original fall.

Sigil of the Fast Miner

Mining fatigue, not really anything else that can be done here.
Mining fatigue reduces attack speed as well, so it should be sufficient (and is actually quite strong).

Sigil of Haste

Slowness, grounded (unable to jump).

Alternative suggestions:

• Added exhaustion
• unable to sprint (no idea how that would be implemented)
• "etsaH"-effect

PLACEHOLDER:

Temp ideas for "third tier":

Sigil of Magnetism

Attracts projectiles/homing in on target

Sigil of Compression

Makes heavy. Ice might break, cannot swim up, "compressed" strikes or similar (would be too op), fires 9 arrows at once, compresses the inventory of the target...

Sigil of Phantom Bridge

Imprison in phantom blocks?
Acts as a shield with infinite durability (that deflects arrows, reflects them or allows you to charge or something else totally not "borrowed" from another mod that might or might not be called the betweenlands mod or redstone arsenal)

No idea.

Sigil of the Nut

Plays something from Nuts soundcloud.

Teleposition Sigil

Teleposes the target to the teleposer saved in the teleposition sigil.

Transposition Sigil

If player: temporarily uses target soul network to power your stuff; if the player doesn't have a soul network... well, you know what happens.

Sigil of Ender Severance

• Deals massive damage to endermen
• is totally useless

Sigil of Supression

Removes all potion effects from the target?
Transfers positive effects to you.
Transfer negative effects from you to the target.

I'd say it's worth accepting that some sigils will not be useful in this context. And most sigils that have an active state should be excluded outright, as they already can be used with the Sentient Sword or any other, in their usual fashion.

Also, lorce's list talks a lot about player effects -- pending a mob (boss) with such powers, I suggest thinking first in terms of what it would do to various mobs. E.g., for the Water Sigil, you might be talking about reduced detection range, slowed movement (except swimmers) perhaps increased vulnerability to knockback. Even for the high-tier sigils, remember balance. For Transposition, mob capture is probably excessive, an Enderman-like forced teleport might not be.

Keep in mind mob capture is something that is not yet available in Blood Magic and it doesn't help you at all in a fight. It doesn't kill the mob. It can, however, be used to build an altar with witches as LP source.

That was an example of sigils not only to be used as pure combat assistance but also to offer completely new possibilities.

"Doesn't help you at all in a fight?" I beg to differ! It lets you instantly remove the mob as a current threat, and (as you note) turns it into a resource instead.

Also, Blood Magic may not have that sort of capture, but it has the Ritual of Containment (I've lately been using that against night zombies, spiders, and occasionally witches, for easy sacrifice) and teleposers to put them into a trap or pull them out. I think using those are much more interesting than instantly capturing a mob on a melee hit.

You're right. Then again, you rarely fight multiple mobs, it is just a suggestion and there can be restrictions added to it. It doesn't help you in a fight regarding "killing mobs to farm" which is why you usually fight mobs.

That being said, TehNut opposes this idea which is why I'll remove it from the list now.
It wouldn't get accepted anyways.

Nut said he's not opposed to welding Sigils to shields (to provide these same effects (or stronger, since you wouldn't be able to use the sigil in offhand)), just to keep that in mind.

This would also give BloodMagic shields that are not boring.

More effect ideas: #1023

#1361, #740 Shield requests / "Suggestions"