What is the adjective form of hell?

There isn't a single, widely accepted adjective form of "hell". However, depending on the context, you could use:

* Hellish: This is the most common and straightforward adjective, meaning resembling hell, or extremely unpleasant.

* Example: "The traffic was hellish."

* Infernal: This word suggests something related to hell or the underworld, and can also mean extremely unpleasant or wicked.

* Example: "He endured infernal pain."

* Diabolical: This word means devilish, evil, or extremely wicked.

* Example: "A diabolical plan."

The best choice depends on the specific meaning you want to convey.