The Burning Summit Rare Grenade Mod is rewarded after anything dies 20 Times in Tyreen's Death Machine from the Side Mission Sellout.
She will add the Burning Summit to your inventory and send you another one by mail.
You can only compete this challenge once per character.
Anything that dies in the machine counts towards this hidden challenge (Zane's Clone or the Psycho that dies during the quest).
Thank you to SSpyR and FromDarkHell for discovering this.