Grease is a phenomenal spell for its spell slot, especially once you get a few Caster Levels under your belt so that it lasts. For many, grease is the go-to 1st-level combat spell after the first few levels.1
There are a number of reasons for this.
Grease has a goodly-sized area, that simultaneously isn’t so large that it’s impossible to avoid your allies with it.
This eliminates one of the more annoying defenses a Wizard may have to worry about, especially later. This means that grease remains good long after you have higher-level spell slots available, unlike color spray and sleep.
Conjuration is widely regarded as the best arcane spell school in the game. It’s got the most or second-most spells of any school2 – and an enormous number of them are very powerful. It’s also fantastically flexible, with a very broad range of effects available to it (battlefield control, like grease, teleportation, summoning and calling effects, even great blasting spells like orb of force from Spell Compendium). This makes it a very good school to specialize in – either literally, as a Conjurer, or simply through feats, prestige classes, and the like. And if you do, grease can benefit from this specialization.
When grease is cast under you, you have to pass your Reflex save to avoid falling in the first place, and then you have to pass your Balance check to move (and in the process once again risk falling!). For such a low-level spell, that’s rather reliable. There are a lot of monsters with pitiful Balance checks out there.
Reliability in the Face of Defenses
Grease hurts your enemies even if they pass both save and check. They can only move at half speed, and more importantly, the Balance skill has this rule:
You are considered flat-footed while balancing, since you can’t move to avoid a blow, and thus you lose your Dexterity bonus to AC (if any). If you have 5 or more ranks in Balance, you aren’t considered flat-footed while balancing. If you take damage while balancing, you must make another Balance check against the same DC to remain standing.
Two things here. One, every attack forces a Balance check versus falling down. Even the Wizard can sit there and hit enemies with ray of frost to force those checks. A ranged touch attack against a flat-footed opponent is very easy – against a lot of foes, it’s AC 10. A Wizard 4 with Dex 14 has a 70% chance of hitting that. A Wizard’s allies can have even better chances, and get more attacks.
Dungeons & Dragons 3.5 Version
And then there’s the fact that Sneak Attack applies against enemies that are flat-footed. That means, no matter how well you do on your Balance check, if you don’t have 5 ranks, you are now very, very scared of the Rogue. You’ve taken a penalty to AC, he’s going to attack you twice, and he’s getting +2d6 or more bonus damage on both attacks. For a 3rd to 4th level character, that is very dangerous. So dangerous that I consider 5 ranks of Balance a high priority for just about every character.
And then you have to make a check against falling down. Twice.
In Pathfinder, only creatures who move through grease are flat-footed. How this works is unclear: the strictest interpretation I can find is that they are flat-footed only for Attacks of Opportunity made during their motion. I cannot find a solid answer on whether a creature who moves through grease is still flat-footed once they stopped (e.g. on other creatures’ turns).
That said, teamwork can take advantage of this. If one player casts grease, another uses bull rush or other effects to move enemies through it, and the Rogue takes Attacks of Opportunity, this can rack up some nice “free” damage. If they remain flat-footed after this motion, the Rogue can really go to town, since Pathfinder does not have the rule about 5 ranks in Balance protecting you the way 3.5 does.
And having a creature stay stock-still in the middle of grease to avoid these penalties is still a great form of area denial.
Grease hits an area with a very reliable form of suck. There are a lot of creatures and characters that are going to be seriously inconvenienced by grease even if they never fall down.
Which means that enemies are going to try to get out of it, and stay out of it. Welcome to area-denial: you can now tell your enemies where they can, and can’t, go. Battlefield control is one of the most powerful tactics in 3.5, and grease is very, very good at it for its level.
1 Before you start hitting their HD caps, color spray and sleep are usually preferred over grease, and after a while the fact that grease is on the ground makes it much less useful because so many things are flying – at that point, though, you’ll be hard-pressed to find many good combat uses for any 1st-level spell.
2 Which school has the most spells depends on books in play; for example, there are more Transmutations in the Player’s Handbook than Conjurations. Not coincidentally, for those who don’t think Conjuration is the best school, Transmutation is usually the school that’s taking that title.