Spell Dice. The hydroid heart's traits are fueled by a special kind of energy called spell dice. The heart can gain spell dice through absorbing spells, unless it has reached its maximum of 10 dice. Each die is a d6. At the start of combat, a heart typically has 5 spell dice.

Bound. Each heart is bound to exactly one hydroid. If the hydroid is destroyed, the heart is destroyed as well.

Explosive End. When the heart is destroyed, it explodes. Each creature within 10 feet of it must make a DC 18 Dexterity saving throw, taking force damage on a failed save, and half on a success. The damage is a number of d6s equal to the heart's remaining spell dice.

Refuel. The heart spends one to three spell dice. If the hydroid this heart is bound to is within 80 feet of it, it regains hit points equal to a roll of those dice.

Absorb Spell. When a creature within 10 feet of the heart casts a spell targeting it or attacks it with a magical item, the hydroid heart absorbs the spell, neutralizing it. The heart gains spell dice equal to the level of the spell, or 2 dice if it was a magic item.

Rebuke. When a creature within 10 feet of the heart makes a nonmagical weapon attack against it, the heart expends 1 to 5 spell dice. The attacking creature must make a DC 18 Dexterity saving throw or take force damage equal to the amount of dice consumed,