Etched Sigil

Etched Sigil is a Legendary Source for the  in Diablo III, added in patch 2.4.0. It requires character level 30 to drop.

The unique affix automatically casts any equipped non-channeled skill that costs Arcane Power with every second (not tick of damage) of a channeling of a skill. It cannot cast two channeled skills. If more than one spender is equipped, it will randomly alternate between them. Spenders will be aimed at the same point where the original skill is cast (at mouse cursor). It cannot cast skills that do not require targeting, and cannot cast with Aether Walker. If is an equipped skill, Etched Sigil will summon it, but will not resummon it until it expires.

As of patch 2.6.1, channeling any skill also boosts damage of all Arcane Power spenders (including the channeled skill itself).

As of patch 2.6.10, the damage boost and free cast only apply to Energy Twister.