Immolation

5th-Level Evocation
Casting Time: 1 action
Range: 90 feet
Components: V
Duration: (C) 1 minute

As in Xanathar's Guide to Everything except the additional damage is considered the burning condition. It has also been specified the flames can only be extinguished by passing the save or through magical means, and the following has been added:

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, both the initial damage and ongoing damage increase by 1d6 for each slot level above 5th.

Source: XGE 158

Changelog

Altered spell to specify the extra damage is the burning condition and clarification was made that the fire can only be extinguished by passing the save or through magical means
Added upcasting option.