This is a dark obsidian like blade, with shadows slowly seeping from it. Once summoned it deals 1d8 + modifier slashing damage. Once per turn if you hit a creature with this blade it absorbs the blood it spills and adds 1d6 necrotic damage. It will continue to increase 1d6 damage for every hit, however every turn without hitting a creature it decreases 1d6 and loses its extra damage if sheathed or if it disappears. The blade increases in size and complexity as the damage increases. 

This sword does work with aberrations and spirit like creatures however other creatures with no blood such as undead or elementals to not increase damage when hit. 

The sword can absorb other magical weapons abilities however, if the weapon requires attunement then you still lose an attunement slot. It takes an hour of concentrating with both weapons for the magical abilities to absorb.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage: Necrotic, True Attunement- Warlock, Hexblade or pact of the blade, Versatile, Sap