That upgrade looks tempting, but naturally things are more complicated than they seem. As you jump across the gap...

...the bars close, and you're left holding onto the ledge. Very carefully traverse to the right, following the active ice vents as they move along ahead of you.

Pull up...

...and step on the pressure pad to extend a drawbridge to your right.

In the co-op game there are 2 pressure pads (plus a third pad near the health upgrade). The one on the right extends the drawbridge. The middle one lowers the bars so you can reach the upgrade, but don't worry about that just yet. Only one player needs to traverse past the vents and extend the bridge.

When you jump onto the drawbridge, 2 Marks of Set appear and enemies begin to emerge. You can close the marks to prevent more creatures from appearing or let them come and kill them for points. this is also a good spot to attempt the Challenge: Kill an Enemy Using an Exploding Mummy's Self-Destruction Attack.

If you shoot the large mummies while they're still standing in a cluster, it's likely that one will explode, destroying the others.

You'll then receive a ring: the FROZEN MARK OF SET'S ASCETIC (Cold Resistance, Defense+, Speed+, Ammo Efficiency+, 10% Cheaper Items).

Then finish off the remaining enemies. In the single-player game, it may help to create a bottleneck by retreating up the stairs to the southwest then shooting/bombing enemies as they approach.

[Part 2 | Return to the Tomb of Hoarfrost Walkthrough]