Void Crystal Weapon – Can be used with any standard or magical weapon and adds the following affect: Once per day, upon hitting an enemy, the sword sends the target to a dark realm until the start of your next turn. You choose where the enemy re-appears within 30ft.
When I think of Void Crystal, the first thing I wonder is where a material like that would come from? The idea of a dark plane encroaching on our own comes to mind. Is this indicative of my own fear of things I cannot control? Or more realistically, a manifestation of the Great Old Ones hour of awakening (praise to our dark lords). Nevertheless, may be place where the vail between our realm and another is partially thin; interesting concept for a dungeon; hint, hint. A Wizard working with powers he or she does not understand, or a cult trying to summon their God into this world. I get a very Cthulhu feel from it all. In these scenarios, all around the portal you can describe dark crystals that seem touched by other side. If you stare into them long enough, you can almost see this realm of void, terror, and an eye that almost catches yours before you look away… Rewarding this item to your party by means of gems or small pieces of the crystal; they can then socket these gems into existing weapons. Another idea is to send the party on a quest to seek a blacksmith that specializes in occult magics to have the material forged into items. Right away you are crafting a strong narrative and lore to accompany this magical item that your players will remember; for good or ill.
Void Crystal also has a bit of cursed feel to me. After the character has spent a long enough time with the weapon, I would not be surprised if something from the other side reaches out; whispering… The creature wants entrance to your realm, but you are no fool like those cultists… perhaps a bargain can be struck? This void entity could have some fun implications for Warlocks or a means to bring Devils from another plane.
When the weapon is used and a creature is temporarily sent into the void, play up the sneak peek into this realm. “A nightmarish hellscape appears, only to suck the creature in! A look of terror flashes across the creatures face as the void envelopes him.” Similarly, I think it would be fun to show panic and confusion as the target re-appears. “The fighter blinks back into this world, wide-eyed and appears to have soiled himself.” Really, whenever possible you should have creatures soil themselves; pro DM tip!