I haven't played D&D for a long time.....

[Shawnis]

....so will someone tell me what spell "Law Domain" does?

[chibibar]

It is your Domain spell list.

Each Cleric who follows a diety get access to Domain spell

check it here.
http://www.d20srd.org/srd/spellLists/clericDomains.htm

[Tulisin]

I've only played D&D a couple times and am vague on the game in general (although I get the jokes, which is nice), but I'm pretty sure the cleric saying "law domain" was a way to justify himself having a spell, not a spell he was casting.

Basically Redcloak was confused that he could cast hold monster, but apparently clerics of the "Law Domain" can.

[Poppatomus]

Clerics that worship a particular deity get access to one or two "domains" associated with that diety. Each domain has a special power, sometimes an additional spell, sometimes a unique spell like ability or stat bonus (the luck domain lets you re-roll one roll per day). additionally the domain provides a seperate spell list of, i think, one additional spell every level that the cleric has access to through the domain. These are often spell that the cleric class would not otherwise be able to cast, or that a cleric would not usually have at that level.

Clerics of the law domain, according to www.d20srd.com cast all of their spells as though they were one level higher as their granted ability. additionally they get the following spells, one at each level.

1. Protection from Chaos: +2 to AC and saves, counter mind control, hedge out elementals and outsiders.
2. Calm Emotions: Calms creatures, negating emotion effects.
3.Magic Circle against Chaos: As protection spells, but 10-ft. radius and 10 min./level.
4. Order’s Wrath: Damages and dazes chaotic creatures.
5. Dispel Chaos: +4 bonus against attacks by chaotic creatures.
6. Hold Monster: As hold person, but any creature.
7. Dictum: Kills, paralyzes, slows, or deafens nonlawful subjects.
8. Shield of Law F: +4 to AC, +4 resistance, and SR 25 against chaotic spells.
9. Summon Monster IX*: Calls extraplanar creature to fight for you.