The sirena crab rider’s innate spellcasting ability is Constitution (spell save DC 13, +5 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

• at will: shape water

The sirena crab rider can breathe air and water.

When the sirena crab rider reaches 0 hp, roll a d20. On a result of 1, the monster is defeated as normal. On a 2-10, the rider is defeated, and the creature is treated as a Giant Crab for the remainder of the battle. On a 11-19, the mount is defeated, and the creature is then treated as a Sirena Thug. On a 20, the two are separated but not defeated, becoming a Giant Crab and a Sirena Thug respectively. The Thug gains the Prone condition and moves to an unoccupied space next to the Giant Crab. If no space is available, the Thug is prone in the Giant Crab’s space and cannot stand up until they are no longer sharing a space.