Kill Draconic Tree Sentinel
To complete this square you need to defeat Draconic Tree Sentinel, the field boss guarding the entrance to Leyndell, Royal Capital. Wait until the "ENEMY DEFEATED" or equivalent screen appears to mark the square.
This boss guards the entrance to Capital and is a field boss, so it can be killed without any requirements. This square has synergies with Kill 5 bosses that ride a horse and Kill 5 bosses with ‘tree’ in their name.
Draconic Tree Sentinel counts as an Altus Plateau/Mt. Gelmir boss.
DANGER
Under Elden Squares rules, you are *not allowed to push him off the nearby cliff.