Elasmobranchian Eruption (Sharkcano)

The temperature rises dramatically as the ground begins to shake.  With a violent blast, a volcano brimming with elementals sharks bursts from the earth at a point that you can see on the ground. The volcano is a 40-foot-radius sphere of earth and stone coated in lava centered on that point. When the volcano first appears each creature in its area must make a Dexterity saving throw.  A creature takes 20d6 fire damage and 20d6 bludgeoning damage on a failed save, or half as much damage on a successful one. 

The volcano's area is difficult terrain that is covered in lava, which ignites flammable objects on contact.  A creature that enters the volcano's area or starts its turn in there must make a Constitution saving throw, taking 6d10 fire damage on a failed save or half as much damage on a successful one.

Each round you maintain concentration on this spell, you can use an action to cause the volcano to gush, making lava flow down its slope and spread out across the ground in all directions, increasing the radius of the volcano's area by 10 feet.

Spells that deal cold damage or produce water that make contact with lava cause the area of lava they touch to cool and harden into solid stone (at least a 5-foot-square section is cooled). Each 5-foot-square cooled lava section has AC 5 and 15 hit points. Reducing a section of cooled lava to 0 hit points breaks it up, causing it to be remelted into lava.  Each 5-foot-section of cooled lava is considered outside the volcano's area until it is destroyed.

A creature occupying a lava-filled space when it is cooled must succeed on a DC 15 Dexterity saving throw.  On a failed save, the creature is petrified until the cooled lava is destroyed.  On a successful save, the creature immediately leaps on top of the newly cooled lava.

Additionally, each round you maintain concentration on this spell, the volcano hurls out magma elementals in the form of sharks on your turn.  Lava sharks summoned by this spell use the statistics of the sharks listed below, though they are elementals instead of beasts, immune to fire and poison damage, vulnerable to cold damage, and possess the following traits:

Heated Body. A creature that touches the shark or hits it with a melee attack while within 5 feet of it takes 3 (1d6) fire damage.

Volcanic Camouflage. The shark has advantage on Dexterity (Stealth) checks made while in lava.

Roll on the following table to determine what appears:

You choose the unoccupied spaces within the volcano's area where they appear.

The lava sharks are hostile to all creatures, including you. Roll initiative for the summoned lava sharks as a group, which has its own turns. The lava sharks pursue and attack the nearest non-lava sharks to the best of their ability.

Lava sharks summoned by this spell treat lava like water for the purpose of their swimming speed and Waterbreathing trait, and are immune to any damage inflicted by the spell.  A lava shark that ends its turn outside of lava, takes cold damage, or makes contact with water is restrained until the end of its next turn. If a restrained lava shark ends its turn outside of lava, it must make a Constitution saving throw.  If a lava shark fails its save three times, it permanently turns to stone and is subjected to the petrified condition. The failures don't need to be consecutive; keep track of them until the lava shark accrues three failures or ends its turn in lava, resetting the count.

When the spell ends, the lava it produced cools and hardens into stone after 1 minute.  Any lava sharks remaining when the lava cools are instantly petrified.