Tarchamus' volcano

History
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. 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 secret library.

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 power protection from fire.