Tarchamus' volcano

Tarchamus' volcano was an extremely powerful spell created by the legendary Netherese arcanist Tarchamus.

The spell harnessed the power of Phlegethos to create a volcano capable of destroying cities. In approximately -3500 DR, Tarchamus used the spell to destroy the floating Netherese city of Tenish. The Mavin's create volcano spell may have been an attempt to recreate Tarchamus' spell, although the two spells functioned differently. In 1478 DR, a group of adventurers including the tiefling Farideh and her sister Havilar used the spell to destroy Tarchamus's secret library. Whether or not the scroll containing the spell survived the cataclysm was unknown.

Spell Level
The exact level and power of the spell were unknown. Given the spell's incredible power and the fact that many powerful Netherese arcanists spent years searching for the spell it was probable the spell was 10th level. However, with the fall of Karsus, Mystral revoked human access to 10th-level magic. An additional factor arguing in favor of a spell level lower than 10th is that the spell was successfully cast from a scroll by the cambion Lorcan who, although he had some magical abilities, was almost certainly not an archmage. The spell may have been castable by modern casters because the scroll was created prior to the fall of Karsus.

Drawbacks
The gate to Phlegethos opened directly under the caster, which made it necessary for the caster to either quickly remove himself from the area or else have extremely powerful protection from fire.