Contortionist.

The fomorian can enter a space large enough for a Large creature without squeezing.

Crawling Stance.

While the fomorian has the prone condition, crawling does not cost it extra movement. In addition, the prone condition does not grant advantage on attack rolls against the fomorian.

Spider Climb.

The fomorian can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Actions

Multiattack.

The fomorian makes two Slam attacks and uses Crawling Hex if it is available.

Slam.

Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage.

Crawling Hex (Recharge 4–6).

The fomorian targets one creature it can see within 60 feet of itself. The target must succeed on a DC 15 Wisdom saving throw or take 31 (7d8) psychic damage, have the prone condition, and become cursed for 1 hour. While cursed this way, the target can't stand up and end the prone condition on itself.

Fomorian Deep Crawler