Volcano.

 You expend all your spell slots and call on your fiendish master for aid, a volcano of demonic power appears and lasts for 5 turns doing a different effect on each one: turn 1 the volcano forms, every creature within a 50 foot radius of a point of your choice  must make a dex saving throw, on a fail they take 5d6 bludgeoning and are knocked prone, on a success they take half as much damage and aren't knocked prone. Turn 2, the volcano shoots out 8 balls of flaming rock you control where these land but they cannot be more than 100 feet from the centre of the volcano each one does 5d6 fire + 5d6 bludgeoning damage over a 20 foot diameter and requires a dex saving throw, the creature/s take half damage on a success. Turn 3, the volcano shoots out a huge cloud of hot toxic ash, every creature within 100 feet of the volcano must succeed a constitution saving throw or take 2d6 poison and 2d6 fire damage the cloud also lowers light level to dim and makes the area lightly obscured as well as killing or harming all plants in the area. Turn 4 lava pours out of the volcano and flows down the side. Every creature within 25 feet of the volcano centre takes 100+5d10 fire damage (a creature with resistance to fire damage takes 100 and a creature with immunity takes 5d10) it also sets any combustible materials in that area on fire. Turn 5 the volcano erupts doing every previous effect as well as 100 bludgeoning and 100 fire damage to every creature within 75 feet of the centre, the volcano then disappears. you can attempt to cast the spell without all your spell slots available, however the DM can decide what might go wrong you may not get all the effects, the effects may not be as powerful or it could backfire and all the energy you pour into it pours straight back into you, disintegrating you instantly. To have all the effects you need to maintain concentration the entire time if your concentration is broken then it ends instantly. The spell takes a great toll on you for 2 days after casting you cannot use eldritch invocations or any pact features as you have called on your master for such a massive favour you also cannot cast the same spell again for a week. “Every creature” does not include you however it does include your teammates and NPC’s. Note, you are not creating the volcano and the magic used is sort of a sacrifice to the fiend therefore in especially sacred areas or on an area that has been affected by the hallow spell with holy power this spell will not work, however in an area that has been affected by the hallow spell with unholy power than you do not have to maintain concentration for the spell to work.