This was just revealed:
The art seems to be inspired by depictions of Hindu Deities. In Hinduism, many arms are used to convey the wide variety of characteristics and aspects a deity controlled, as well as their power. Here, since this character has nothing in their hands besides Abyssian Fire, it may represent the character’s desire, and power, to obliterate everything. If you follow the lore, especially the Abyssian sister’s Ciphertext story, this is especially relevant since the Abyssian are shown to be working against the wished of the Tree of Eyos, seeking to destroy it before it destroys them.
Judging by the art, and the fact that all previous grandmasters had something to do with Generals, I predict that this minion would be something like:
You may use your Bloodborn Spell as any number of times on each turn.