I am the DM in a campaign in which one of my friends plays an ardent. Everyone agrees with my friend that his ardent's Wormhole Plunge power kills any enemies that go into it. Is this true? It's annoying when he kills bosses in one hit.
It used to one-hit kill non-flying enemies who entered the zone, but that was ridiculous so it was errata-ed.
Wormhole Plunge creates a zone. Any time a creature enters the zone, the ardent can teleport it as a free action. Because teleportation, unlike forced movement, does not prevent you from putting the target in midair, ardents could teleport the target to a square above the zone. The target would then fall back into the zone, taking falling damage and re-entering the zone so the ardent gets to teleport them again. Repeat as many times as needed until the target is dead.
Because Wizards of the Coast tried pretty hard to fix any powers that allow one-hit kills, Wormhole Plunge received errata in May of 2010 that limited the ardent to only being able to teleport a creature in the zone for free once per round. It is highly recommended that you use the errata when playing 4th edition D&D, as Wormhole Plunge is hardly the only power or combo that was broken when originally published.
If your players are strongly resistant to fixing the power with errata, try using some enemies who can teleport the PCs to inflict large amounts of falling damage. Most players will agree to give up one-hit kill mechanics if there's a high risk of monsters using them against the party. Every 4e group I've played in has eventually house-ruled teleportation to not allow you to teleport foes to places where they'll take falling damage; it's just too easy a way to deal damage, both for the PCs and for the monsters.
Wormhole plunge does a few different things.
First, on a hit against AC, Wormhole Plunge does Weapon+Cha mod damage. On a miss against AC, Wormhole Plunge does half damage.
Second, you can teleport the target Wis mod squares. If he ends this teleport adjacent to an ally, that ally can make a melee basic attack against the target.
Third, a zone is created in the space that the target started in. This zone lasts until the end of your next turn. Once per round as a free action, you can teleport a creature standing in this zone 3 spaces.
Fourth, at the start of each creature's turn who is within 3 squares of the zone is pulled 1 square towards the zone.
None of these things instantly kills an enemy who ends up in the zone.