The first new skill for each weapon type is available at combat level four, second at level seven, and third at level ten.
Skills for each weapon type are as follows:
Swords
quick slash in all four directions
quick slash in all four directions plus magic projectiles in four directions that do 3x weapon damage
quick slash in all four directions twice plus magic projectiles in eight directions that do 6x weapon damage
Clubs
power slam plus a single lightning projectile that does 2x weapon damage
power slam plus three lightning projectiles that do 3x weapon damage
power slam plus five lightning projectiles that do 6x weapon damage
Daggers
quick lunge then rapid stab attack then quick retreat
quick lunge then rapid stab attack plus three throwing daggers that do 3x weapon damage then quick retreat
quick lunge then rapid stab attack plus five throwing daggers that do 6x weapon damage then quick retreat
Here's a video showing these attacks (max level) in comparison to the vanilla special attacks:
And here's a video showing me clumsily defeating a bug boss with them:
By default these skills override the existing skills once you reach the required level. You can edit the config.json for Advanced Melee Framework to alternatively require holding down a modifier key to switch to this mod's special attack.
Technical
Requires SMAPI and Advanced Melee Framework (which uses Harmony).
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.