No defined shape. The Accumulation is just a mound of insects, and so therefore it can occupy the space of others.

Hydrophobia. If the Accumulation grapples a creature and that creature makes contact with a substantial amount of water, it no longer has the grappled condition and the Accumulation moves 1d6 ft away from it. The Accumulation now has the Frightened condition.

Pyrophobia. If the Accumulation grapples a creature and that creature takes fire damage or drinks/is under influence of a Hellish healing potion, the Accumulation takes 50 (10d10) fire damage.

Ear-splitting screech. The Accumulation emits a powerful screech that rends the mind itself. Wisdom saving throw:  DC 11,  all creatures in a 25 ft radius. Failure: 10 (2d10) psychic damage.

Ten thousand bites. The Accumulation turns from a humanoid shape to a long carpet of insects that crawl up your legs and bite you everywhere they can reach. Dexterity saving throw:  DC 15, 1 creature it can see within 35 ft. Failure: You are now grappled by the Accumulation, and you take 18 (6d6) piercing damage at the start of all your turns whilst you are grappled by it. Whilst you are grappled by the Accumulation, you can still move.