"Avatars" are sort of like commanders in mtg, but you don't ever cast them into play, and also they set your life for the game (it can vary by avatar). sometimes they give unique win conditions.

instead of casting your commander like in magic, you instead spend energy (mana) to put energy on them. you can then use this energy to use their effects.

avatars can only be energized by 3 of the 6 main colors, so they only fit with certain deck color compositions.

they generally have a generic card draw effect, which basically allows you to never waste mana,

this avatar, "Yakub" is pretty good. he can transmute energies (mtg lands) into entities.

the reason this is such a powerful ability is that because there are no dedicated energy (land) cards, you can just play your best card as an energy, then transmute it from the energy (land) zone, into the entity (creature) zone, even if it's a structure or meme.

another reason why this is power is that you could play without any explicit entities (creatures) in your deck, and instead just rely on transmuting non-entities from the energy zone, or frenning together non-entities into the entity zone, as an entity.

this a way to cover a deficit in the fire element during deck construction