The Goal of All Life is Death

You utter a forbidden phrase that reveals the ultimate destiny of all living things, which is to perish. A spectral timepiece materializes behind you with its hands ticking with the finality of fate, marking all creatures of your choice within range for imminent demise. Each round on initiative counts 20, 15, 10 and 5, the clock advances, gaining one charge of Momento Mori. When you cast this spell you must pass a stealth check DC16 or have the Subtle Spell metamagic or become the target of the creatures you marked.

When the clock reaches 12 charges, or your concentration is broken, energy bursts from the clock as it explodes in a devastating wave of Necrotic energy. Although creatures marked by this spell must pass a Constitution Saving throw DC equal to your spellcasting ability score. 

On a failed save, a creature takes 12 psychic damage plus an additional 1d12 necrotic damage per charge of Momento Mori that was expended in the blast, or half as much on a successful save. Additionally, creatures that fail the save suffer additional effects based on the number of charges expended at the time of the explosion. These effect lasts until healed by either the Greater Restoration or Wish spells. The effects apply all effects lower than or equal to the number of charges expended.

• 3-5 charges. Creatures gain disadvantage on Death Saving Throws and all healing they receive is halved.
• 6-8 charges. Creatures that don't have resistance or immunity to Necrotic damage gain vulnerability to Necrotic damage. 
• 9-11 charges. Creatures gain 3 levels of exhaustion.
• 12 charges. All marked creatures are reduced to 0 hit points and cannot be revived by anything except for a Wish or True Resurrection spell.