Immersive Ink

Reminder what the DM said: A funny-looking creature ambushes the party from the south. I assume it was in the tunnels and followed us into the room.

This would be an appropriate time for Kiplyn to say "I told you so!"

Round 1

The fast-moving rust monster wins initiative and continues to target Wilbur. It taps Wilbur's hauberk with its antennae, and the chain links immediately begin to rust, at an almost supernatural rate. (Wilbur's damaged armor is about as effective as leather.)

Willie's plan works! The hungry rust monster takes the bait, and chows down on the iron spikes. This allows our heroes to retreat 30' to the north (into the center of the room) and target the rust monster with missile fire from a safe distance.

Bilfur nails it right between the eyes with a perfectly aimed sling shot, and Kiplyn and Willie also connect with their sling stones. Wilbur's dagger finds its mark, but then immediately begins to rust! Dougal and Dame Simonetta miss with their attacks.

The rust monster is badly wounded, and got what it came for: a bellyful of rust. It flees back into the southern tunnel to hopefully digest its meal in peace.

Do you pursue the fleeing rust monster? Do you attempt another round of missile fire as it goes?

Rust monster wins initiative 3-1
2 attacks vs. Wilbur: 4 miss, 16 hit
Rusts his armor, penalizing him to AC6.

Party counterattack (missile attacks at +2 due to short range)
all slings except dagger for Wilbur
Bilfur: 22! hit for 4hp
Dame Simonetta (+1 DEX): 13 miss
Dougal: 11 miss
Kiplyn (+1 DEX):19 hit for 6hp
Wilbur: 16 hit for 2hp
Willie (+1 DEX): 20 hit for 3hp

combined damage: 15hp

Rust monster lost more than half its hit points, so it rolls morale: 2d6=5, flee!