Multiattack. The amphisbaena makes two attacks, only one of which can be a constrict attack.
Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit:11 (2d6 + 4) piercing damage.
Constrict. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit:13 (2d8 + 4) bludgeoning damage, and the target is grappled (escape DC 16). Until this grapple ends, the creature is restrained, and the snake can't constrict another target.