I'd make him your baseline for the fodder rat monsters in this dungeon. Urous is comparable, but perhaps a bit weaker, than my goblins.

Here's a thought. Give them all Dark Vision instead and drop a Blackout spell on them. That gives -10 to hit.

I'm not going to look for the rules right now, but you can give creatures a spell-by-spell advantage like the Wraith. However, there's also Obscure (Vision, Defensive, Limitation: Doesn't work against Dark Vision) at 10 levels on page B71.

In addition to Obscure or Blackout spell, I'd also give Urous a breath attack (toxic/poison/acidic or nausea affliction) and perhaps Body of Shadow, which would play into his ability to create darkness. That's just me.

Here are some rules for fighting in total darkness:

Visibility (B394)

Total darkness is -10. Regular darkness is up to -9. Night Vision doesn't help against total darkness (-10).

If the attacker (including his weapon) is invisible or in total darkness but the defender is aware that he is being attacked*, he may dodge at -4. If the defender makes a Hearing-2 roll, he may also parry or block—still at -4.

If he is completely unaware of his attacker, he gets no defense at all!

*For first attack, if they are being ambushed, make a secret Quick Contest roll between the rat's Stealth and the victim’s hearing. The rats are -5 if they moved faster than Move 1, and they probably will.

If the attacker is in smoke or unnatural darkness, but the defender is standing outside of it, he defends normally, since he can see the attack coming.

In close combat, attacker cannot see his foe, but knows his location for sure. If engaded in close combat, use the rules above, but no Hearing roll is required and the attack penalty is only -4.

Attacking

If the attacker is in total darkness, he can make a Hearing-2 roll – or use some other method – to discover his foe’s location.
Wild Swing (B388-389)

In Total Darkness, for melee, attacks are a Wild Swing. Roll to hit is -10 and can't exceed 9. Always roll on random hit location table. You can combine a Wild Swing with an All-Out Attack, but you may not choose the “Determined” option to get +4 to hit to offset the Wild Swing penalty. You can also make a Wild Swing during a Move and Attack; use the more severe penalties of the two.

Shooting Blind

If you have a ranged weapon, you may attack someone in total darkness by “shooting blind.” Use the rules for Wild Swings; the penalty is -10 and your effective skill cannot exceed 9 after all modifiers. You cannot take the Aim maneuver.