It's a Trap

Suspecting the teleportation ring would lead them to the arrival chamber of the Tower of Acererak, the party continued the pursuit anyway.

Arriving in the chamber, they were quickly surrounded by a dozen putrescent zombies. Much like the ones in the graveyard in Pontylvar, they would explode when slain.

Standing in one of the doors was Simon, laughing at them. “Welcome to the place of your rebirth, as my servants.”
With that, he targeted one of the putrescent zombies with his rod and launched a cloud of daggers at it, continuing to laugh, Simon closed the door behind himself as he headed up the stairs.

A split second later, the daggers ripped the zombie to shreds and it exploded, setting off a chain reaction, causing them all to explode, bathing the room in necrotic damage.

Fortunately for the party, they saw what was transpiring with enough lead time to get Leenia to reinsert her rod into the pedestal and transport the party away, in the nick of time.