Altar of Tiamat

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

The altar makes one breath attack per round. It cannot make the same breath twice in a row. 

Acid Breath (Recharge 5–6). The dragon exhales acid in a 30­-foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 49 (11d8) acid damage on a failed save, or half as much damage on a successful one.

Lightning Breath (Recharge 5–6). The dragon exhales lightning in a 30­-foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 49 (11d8) acid damage on a failed save, or half as much damage on a successful one.

Fire Breath (Recharge 5–6). The dragon exhales fire in a 30-foot cone. Each creature in that area must make a DC 15 Dexterity saving throw, taking 56 (16d6) fire damage on a failed save, or half as much damage on a successful one.

Poison Breath (Recharge 5–6). The dragon exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 15 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.

Cold Breath (Recharge 5–6). The dragon exhales an icy blast in a 30-foot cone. Each creature in that area must make a DC 15 Constitution saving throw, taking 45 (10d8) cold damage on a failed save, or half as much damage on a successful one.