Blizzard
The Formless Void mostly exists for the purposes of flexibility, which is why all roles can use it. It requires a lot of planning, it requires you to coordinate and know who you plan on borrowing what Essence from, and for what situation, and make sure that the cooldown incurred doesn’t interfere with whatever the next situation you have to manage is.
As a note, you can also borrow something from a player of the enemy faction. The targeting is very unrestricted other than the obvious - it has to be a player, they need to have an Essence currently slotted that is valid to copy (fully passives Major Essences are ineligible).