Kill Sewer Mohg

To complete this square you need to defeat Mohg, the Omen, the boss in the Underground Roadside area (the Capital sewers). Wait until the "ENEMY DEFEATED" or equivalent screen appears to mark the square.

This boss is inside the Capital, so you need to defeat two Great Rune bosses to gain access to him.

Mohg, the Omen counts as an Altus Plateau/Mt. Gelmir boss.