"Well, we released a dozen specimens, all various types of crustaceans: crabs, lobsters and a few more exotic types. Shaal explains. "To do their experimental nature, I don't imagine they would have all survived between the latent energies within them and the usual risks of predation.
Renya knows that tritons and sea elves have communities in the lake though no one knows precisely where. Shaal is either using the term merfolk loosely to mean dwellers beneath the water, or there is a community of half-human, half-fish people in the Nyr Dyv formerly unknown to you.
Below
Arres's astral arms spring forth from his body and the water ripples with the force given off. The crab clearly feels it and it spurs it on to take down this unexpectedly dangerous prey. Arres catches one of the few weak spots on his shell and gets in a solid hit but the crab is undeterred. Its claws lash out and clutch Arres on both shoulders crushing deeply into his torso. He also feels the waves of cold radiating off this creature sink deep into his bones. (22 damage total and you are now grappled.)
Sliven, take your round one actions and Arres can take his round 2. Sliven is close enough that he can close to melee with the crab.
Rolls
Crab Dex Save - (1d20+3)
(1) + 3 = 4
Crab Attack Claw 1 - (1d20+6)
(19) + 6 = 25
Crab Attack Claw 2 - (1d20+6)
(20) + 6 = 26
Claw Damage 1 - (1d8+4)
(4) + 4 = 8
Claw damage 2 Crit - (2d8+4)
(36) + 4 = 13
Cold Aura - (1d4)
(1) = 1