I'm struggling with the spawning conditions of my new boss mob.

Hey everyone! I hope your day is going great.

I've created a boss mob, and I have a spawning condition in mind but I can't seem to figure out how to implement it; is it possible to make a mob spawn upon completion of the end portal? So that as soon as the last eye is placed and the portal lights up, it would summon this mob who would immediately attack you.

Thank you and have a nice day!