This may not require the full multiple phase fight with multiple floors, just some basic phase 1 combat mechanics may be good but I'm going to upload the full design just for clarity sake

The Infected Guardian Boss (name pending, potentially “Heart of the Infection” but would prefer something snappier) is the first major boss battle of Aether II’s dungeon content. It presents a significant step-up in complexity of behaviour and interaction with the surrounding environment compared to other mobs, featuring multiple phases, each with multiple types of attack and behaviours, and a special mechanic for interacting with the arena itself.

The boss starts immobile in the arena, rooted into the ground. The battle begins with the boss breaking the roots that bind it to the floor in an explosive animation that shakes the surrounding ground. The boss then shrieks loudly. Boss music kicks in, then the boss begins to move around, targeting nearby players with attacks.

During this first phase, the boss is incredibly slow to move and turn, but hits incredibly hard. It mostly relies on summoning other mobs during this phase to put pressure on players, and summons front facing defences to block damage. These behaviours force players to evade heavy hitting attacks, deal with additional weaker mobs, and navigate the arena to attempt to hit the boss from behind.

During the first phase, the boss has a large hammer for an arm, if it finds a player near its front, it will raise its hammer above its head then quickly swing in an arc around its front, hitting any mob in that arc with heavy damage and knock back. Clever players may be able to get the boss to hit its own allies with this attack with quick repositioning.

During both phases, the boss has the ability to stamp the ground and summon a wave of thorny roots and vines which project forwards at high speed, damaging and slowing entities that get caught inside.

During the first phase, the boss is able to shoot spore clouds into the arena, which settle and turn into spawner mushrooms. If not destroyed, these mushrooms eventually turn into hostile mobs.

During the first phase the boss will occasionally stand still and start to summon shield spores around it, the player is able to stop this process by dealing enough damage to the boss during its animation, if that fails, the boss will create a shield of spores that protects its front from damage for a duration.

Once the boss receives enough damage, its fungal arm will break off and the visible vegetation will fall away. Visibly weakened, the boss will plunge its hammer arm into the ground and summon up more fungus, which consumes much of the remaining healthy wood that makes up the body of the boss and grows new fungal formations. The boss then rips its arm out of the ground to reveal a large fungal blade in place of the hammer. With another loud shriek, the second phase of the fight begins.

During the second phase, the boss becomes more mobile, it is able to run forward at a relatively high speed, but cannot turn easily unless it stops. Being unable to summon additional mobs during this phase, the boss is now the primary threat and point of attention for the fight.

The boss’s primary attack during the second phase is a three step slashing move, first it quickly attacks the area immediately in front of it, then does a swiping attack that affects the area immediately in front of it, then a spinning attack that affects the entire area around the boss. With each swipe, the boss moves forward, but cannot change direction once the attacks start.

The boss’s final attack is a ranged attack where it plants its arm into the ground, then leaps backwards away from its target, fungal growths then sprout from the ground and throw saw blade mushrooms in an arc towards the targetted player with a small amount of vertical variation. These projectiles deal a heavy amount of shield stun, so are best avoided rather than blocked.

A central mechanic of this boss fight is the decay of the arena, as the boss moves around, does attacks, and takes damage, it will spawn blankets of spores which eat away at the wood of the arena. If the player is unable to destroy these blankets of spores, then holes will start to appear in the arena floor. The boss is able to create small temporary fungal blocks to stop it from falling into these holes, but the player and other mobs are able to fall down.

The boss arena consists of several floors, starting at the uppermost floor. If a player falls through a hole in the floor of the arena, they will land on the next floor down. Each floor has different environmental features, hazards, and layouts. Once a player has fallen into a lower floor, the boss will destroy the floor above and land on the next floor down, continuing the fight on the lower floor. Each consecutive floor grants the boss a series of buffs that make the fight more intense, incentivising the player to remain on the upper floors by removing spore blankets and replacing holes in the arena with their own blocks.