Stumped over an unlock

https://preview.redd.it/9x4hu6wfvvkg1.png?width=757&format=png&auto=webp&s=7ca41e6e05528b4605d6c30bdd0180cd21863c7a

I've been trying to avoid spoilers, but can someone tell me how to unlock this guy? I've unlocked act 3 already, and am clearly missing something simple
tyvm