Kill Putrid Crystallian trio

To complete this square you need to defeat the Putrid Crystalian trio boss inside Sellia Hideway. Wait until the "ENEMY DEFEATED" or equivalent screen appears to mark the square.

This boss is available from the start of the game but is in a secluded, hidden place in Dragonbarrow. The square has synergy with Kill 3 duo/trio bosses.

Putrid Crystallian trio counts as a Caelid/Dragonbarrow boss.