Multiattack. The regisaur makes one Bite attack and one Tail attack. The regisaur can't target the same creature with both attacks.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 47 (6d12 + 8) piercing damage, and the target has the grappled condition (escape DC 18). Until this grapple ends, the target has the restrained condition, and the regisaur can't Bite another target.

Tail. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 26 (4d8 + 8) bludgeoning damage.

Swallow. Melee Weapon Attack: +13 to hit, reach 5 ft., one Huge or smaller creature grappled by the regisaur. Hit: The regisaur swallows the target, and the grapple ends. A swallowed creature has the blinded and restrained conditions, it has 3 against attacks and other effects outside the regisaur, and it takes 7 (2d6) acid damage at the start of each of its turns. The regisaur can have up to two creatures swallowed at a time. If the regisaur takes 25 damage or more on a single turn from a swallowed creature, the regisaur must succeed on a DC 16 Constitution saving throw at the end of that turn or regurgitate the creature, which falls in a space within 5 feet of the regisaur and has the prone condition; the creature no longer has the blinded and restrained conditions.