The thessalhydra's heads may each attack independently. On a crit, the head lifts the target and places the victim in the main mouth unless the victim succeeds at a DC 12 Dexterity or Strength check. The pincer tail does the same on any hit unless the victim succeeds at a DC 14 Dexterity or Strength check.
When a head strikes successfully, it inflicts 2d4 (exploding) damage, regardless of whether it successfully conveys a target to its main mouth. Likewise, the tail pincer inflicts 2d6.
A victim placed in the main mouth takes 2d10 damage automatically per round, and at the end of any round spent in the mouth is swallowed unless succeeding at a DC 15 saving throw. A swallowed victim takes 4d4 (exploding) stomach acid damage per round. A thessalhydra cannot put another victim in its main mouth if a previous victim hasn't yet been swallowed.
A head (AC -7) may be targeted and is severed if dealt 10+ points of slashing damage by a single attack. When/if all heads are severed, the body's AC becomes 4. Severing a head does not reduce the thessalhydra's hit points, but is an effective tactic for bringing the thessalhydra into a less menacing state that can more easily be defeated.
The eight heads and the tail pincer have effectively a 10' reach but may target closer enemies.
There is a 30% chance that a defeated thessalhydra's stomach contains gems, coins, and small valuables of worth summing to 50% of the rolled Hoard treasure.