Invoke the realm of the bat spirits. Summon a swarm of demonic bat spirits that fill a 5ft x 5ft cube. Once per turn you can have them take movement or attack.
The Bats last for 1d7 rounds and disappear at the end of last round. The bat spirits are intangible and can only be harmed by radiant damage. The bats are considered undead.
The bats may move up to 30ft per turn. They inhabit a space another creature is in to attack. The bats make a d20 attack roll adding your spell casting bonus to the roll. The bats deal 15d8 piercing damage. A creature must succeed on a DC18 con save or be inflicted with bleeding and poisoned, each dealing 2d6 damage at the start of their turn for 2d4 rounds. On a success the damage and amount of rounds is halved rounded down.
You can cast this spell as a ritual requiring multiple spell casters capable of 7th level spells(min. 2 more), a casting time of 10 minutes or more (cast in 10 minute intervals), and a living sacrifice who's value is equal to the amount of bats you want to conjure. The spell then creates a swarm of bats equal in power to the amount of spellcasters, equal in duration to the length of the ritual, and equal in quantity to the value of the sacrifice.