Vampiric Blood Summons

The vampirism within you grants you the ability to create undead like yourself, but at a price. Once per long rest, you can roll a number of hit dice equal to your level divided by two. For every two hit dice you spend, you may summon one CR 1/4 skeleton or zombie from a corpse nearby, if available. Also, for each hit die you spend you take 1d4 necrotic damage, ignoring resistances and immunities.