Multiattack

The son of Fenris makes one bite attack and one slam attack.

Bite

Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 5 (1d10) poison damage, and the target is grappled (escape DC 18). If the target was already grappled, it is swallowed instead. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects from outside the son of Fenris, and it takes 28 (8d6) acid damage at the start of each of the son of Fenris's turns. It can swallow only one creature at a time. If it takes 45 damage or more on a single turn from the swallowed creature, it must succeed on a DC 17 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the son of Fenris. If the son of Fenris dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Slam

Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) bludgeoning damage.

Acid Breath (Recharge 5-6)

The son of Fenris exhales acid in a 60-foot line that is 10 feet wide. Each creature in the line takes 45 (10d8) acid damage, or half damage with a successful DC 18 Dexterity saving throw.