To imbue a weapon you first have to find a crystal. These will be rare loot drops and/or something you can craft. Tinkers can use crystals to add powerful effects to weapons. After you’ve placed a Tinker building and assigned a Dwarf to be the Tinker, you can imbue weapons!
With each hit, there is a chance the imbued weapon will create an effect.
Red crystals add a flame effect. When hit, a monster will sometimes get caught on fire. The monster does not take damage from the fire however. Instead, after a short period of time, he explodes, damaging all nearby monsters and itself. If the monster is killed before the detonation, the effect dies with it. This means if you set a monster ablaze, it’s best to stop fighting it and attack another monster, letting the explosion detonate.
Blue crystals add frost. Frost weapons can freeze a monster in place.
Purple crystals add Drain. A weapon with the Drain effect will do less damage, but have a chance to return some hit points to you with each hit.
Green crystals add Acid. A monster who is affected by an acid weapon will take additional damage. If a monster is holding a weapon, it may have its damage reduced as the acid damages the weapon.
Yellow crystals (not shown in the video) add lightning. A lightning weapon does more damage to monsters in armor and has a chance to arc, hitting another nearby monster.