The Soulstone of Netherbane grants the character wielding it a special ability tied to their Eldritch Blast. Once per turn on the first successful hit with Eldritch Blast, the target must make a DC 13 Dexterity saving throw. If the target fails the save, shadowy tentacles emanating from the blast reach out and ensnare the target, causing their movement speed to be reduced to 0 until the end of their next turn.

Wondrous item, rare (requires attunement by a warlock)

When you use Eldritch Blast option while attuned to the figurine, you can momentarily curse one creature targeted by the spell; choose one ability score, and until the end of your next turn, the creature has disadvantage on ability checks and saving throws that use that ability. It can be used as many times as the Warlock's proficiency bonus per Long Rest