How do YOU do spells that affect "up to X HD of creatures"?

[Kerrick]

I've been looking over the spells, and I noticed that a bunch of them say "affects x HD of creatures" or "affects one creature up to x HD". How do you adjudicate it? Do you look up the HD for every creature that could be affected, or simply house rule it to affect x number of creatures, or something else?

 

[Runestar]

The DM just checks how many HD the creature affected has, and determine if the creature will be affected. The player may or may not know how many HD it has, so he has to cross his fingers.

This question is fairly vague. Do you have a specific spell in mind?

 

[Kerrick]

Not A spell, but a group of spells: scare, sleep, deep slumber, scintillating pattern, circle of death...

 

[Monsterknuffel]

When i am DM i look up the HD of my monsters, if they are affected.
If the PCs are lvl 1-3 and the wizzard casts scare, i know that none of my goblins has more than 6 HD and is affected by the spell.

If you ask that questions as a dc, you just have to make a difference.
Players with not much experience normally dont know the HD of the monstes. But if it´s a new spell because are able to prepare spells of a new level, you should know that there is a chance they will succeed. (the spells)
If you are an experienced, long time player you know the HD of many monsters. because you had to look for new races, etc.

But the spells normally do exactly tell you what to do.
Scare: All monsters with less than 6HD (As DM you just look if your monster has more or less than 6HD)

sleep ,deep slumber, scintillating pattern or circle of death give you a maximum (summarized) number of HD affected.
Example: Circle of Death would be able to kill 10 HD of monsters. Monsters with less HD first.
You cast them on the enemys: 3 Goblins with 1HD and 3 Orcs with 2 HD
and 1 Ogre with 3HD
The spell affects monsters with the smallest number of HD first.
Now you just have to summarize.
3*1HD + 3*2HD = 9HD The Ogre, because he has 3 Hit dice would be affected as the third group. But 9HD + 3HD = 12 HD That would exceed the maximum summarized number. So, the Ogre is not affected by the spell.
Hope this helps you, but the short answer already gave Runestar. The DM has to look up the Hit Dice and the Player, if he doesnt know the Monster´s HD, just has to cross his fingers that his spell will work ^^

 

[Gilladian]

This is also the perfect time for the PC to ask for a knowledge check. If he or she wants to be sure their spell will affect the monster(s), then he can check his or her dungeoneering knowledge or survival or spellcraft, or whatever skill you feel is appropriate.

I'd set the DC at 10 for common monsters, 15 for unusual ones, and 20 for special/unique or very rare creatures. YMMV.

 

[el-remmen]

This is also the perfect time for the PC to ask for a knowledge check. If he or she wants to be sure their spell will affect the monster(s), then he can check his or her dungeoneering knowledge or survival or spellcraft, or whatever skill you feel is appropriate.

I'd set the DC at 10 for common monsters, 15 for unusual ones, and 20 for special/unique or very rare creatures. YMMV.

Right, but even a successful check does not guarantee accurate knowledge. I mean, sure it might tell you that generally goblins are weak enough to be affected by sleep - but if a specific goblin leader has more HD or class levels, well then the only way to know (aside from guessing due to ability) is to cast the spell and hope for the best.

At least in my case, I avoid all meta-game language in descriptions.