Well, this is hilarious...
The rats will swarm out, two of them getting in the hallway and attacking the person they notice first, the person straight in front of the door: Romana. Another two rats will remain in the room, but will still attack some party members diagonally through the opened door: one on Devin and one on Kanyil.
One rat bites Romana for 7 points of damage, but the other one fumbles, jumping on Romana's quarterstaff: Romana can make un undefendable melee attack on that rat. A rat also bites Devin for 4 points of damage.
Berryn is up next and can make his way onto the map.
Rolls
Rat's DEF: 2 - (1d20)
(20) = 20
Rat 1 attack (Romana) (CTN: 6 +4 size. Total CTN: 10) - (1d20)
(7) = 7
Romana DEF: 8 - (1d20)
(10) = 10
Rat 2 attack (Romana) (CTN: 6 +4 size. Total CTN: 10) - (1d20)
(20) = 20
Romana DEF: 8 - (1d20)
(19) = 19
Rat 3 attack (Devin) (CTN: 6 +4 size. Total CTN: 10) - (1d20)
(4) = 4
Devin DEF: 9 - (1d20)
(18) = 18
Rat 4 attack (Kanyil) (CTN: 6 +4 size. Total CTN: 10) - (1d20)
(12) = 12
Kanyil DEF: 9 - (1d20)
(8) = 8