What better way to start a new feature than with the alphabetically first monster in the Monster Manual? Sorry, aarakocra.
In case you're not familiar with this D&D mainstay, the aboleth is a vile monster that lives underwater and has telepathic ability. Think Aquaman but infinitely more loathsome. They first appeared in the titular forbidden city of adventure I1, Dwellers of the Forbidden City (along with yuan-ti, tasloi, and mongrelmen, some of whom worshipped the aboleth as a god).
With their mind control powers, it's easy to narratively justify pretty much any ally for an aboleth. While their servitors are obviously tailor-made for them, the kuo-toa are also known to hang out with aboleths and share their aquatic proclivities.
It's often fun to take a campaign into an exotic setting from time to time, and a watery environment is an excellent choice. Aboleths are among the nastiest enemies you can fight in the water.
The mantra of any artillery monster is, "Stay away from the PCs." Conveniently for the slime mage, every attack it can make can potentially hamper enemy mobility, making it that much easier to stay away. A general game plan might be to start with slime burst to hold PCs in place, then move 10 squares away from the enemy to throw slime orbs from as much safety as possible. With luck, allies will daze a PC or two, in which case dominate can come into play. Slime Mages should daze enemies with their own tentacles only as a last resort, because 2-square reach isn't remotely far enough away for safety.
When facing a significant ranged threat (such as any well-balanced party), the slime mage should seek cover. If terrain-based cover isn't available, an aboleth worth its repulsive salt will hide behind its minions or other allies. Obviously it will also direct its allies to close with any particularly threatening ranged attackers. Finally, bear in mind that the two most threatening ranged PC classes, the warlock and ranger, can place their damage-increasing conditions (warlock's curse and hunter's quarry, respectively) only on the closest enemy, so the slime mage should do absolutely everything in its power to not be that closest enemy.
Things really go wrong (from the disgusting monster's perspective) when PCs bust out their best mobility tricks. Fey step, expeditious retreat, and cloak of shadow are just a few of the ways PCs might circumvent the aboleth's protective wall of disposable minions. When this happens, the aboleth's best bet is to get into the water. PCs have lots of ways to break the normal movement rules once, but it's hard to find a PC that can keep up with a swim speed of 10 over several rounds.
For a particularly nasty surprise, have your slime mage retreat into an area with a trap, hazard, or even another group of previously unseen minions. If it's been doing its job as an artillery monster, the PCs will be angry enough to chase after the ASM, which is the perfect opportunity to get the PCs into an even worse situation with some of their encounter powers already spent.
Most artillery monsters benefit from difficult terrain. In this respect, the slime mage is somewhat atypical -- mucus haze means the slime mage is always surrounded by difficult (and repulsive) terrain.
In selecting a lair, the slime mage should look for places with a single choke-point entrance, the better to catch multiple PCs with its slime burst ability on the first round.
On land, most aboleths have below average speed and are large, which is bad news for slime mages trying to avoid melee. In their native habitat, however, any aboleth can swim circles around any of the PC races. Always consider watery terrain, or at least an area of the map with water, in which to locate your aboleth. A mixture of water and dry land with little cover is probably the most advantageous for aboleths. The PCs will be stuck with nothing to hide behind while the slime mage can act as a de facto lurker, submerging every other round after it attacks, then spending its next turn emerging and attacking.
For specific maps, look at the cave map from King of the Trollhaunt Warrens. This is a simple but moderately effective aboleth lair. It has good choke points at the entrances and a river that could provide an escape route or submerging space.
There aren't many preprinted maps with much water on them, so you may need to improvise. One fantastic example of a map that can be adapted is the Commons map from Demon Queen's Enclave. Simply tell your players at the beginning of the encounter that the curving walls form the edges of pools of water instead of rooms. For that matter, you could flip it around and make the enclosed areas the only "dry" land. Assume that the watery areas are connected by underground tunnels and your aquatic monsters can pop up pretty much anywhere on the map, guaranteeing a memorable battle.
Finally, it's worth noting that aboleths, their servitors, and kuo-toa all have darkvision. It's always challenging to face monsters you can't see. Just don't overuse this trick -- it can get frustrating for players.
Aboleth lashers are an obvious choice but still an excellent one. Their tentacle attacks provide a handy way to daze enemies as a setup for dominating by the slime mage. Brutes of Large or bigger size are also great for interposing themselves between PCs and back-rank monsters such as the slime mage.
A mind flayer infiltrator's mind blast attack is a great way to daze several targets at once. You may want to advance the infiltrator a couple of levels to bring it up to the appropriate threat level.
If you plan to use the "escape to another threat" plan mentioned above, some sort of trap can serve really well. An Altar of Zealotry (DMG, pg. 92) dedicated to an obscure Far Realms power would be thematically appropriate to an aboleth encounter (though again, you might need to advance it a couple of levels).
The aboleth lasher is a pretty standard Large brute, but as noted above, it has excellent synergy with its slimily educated brethren.
The aboleth overseer is an excellent "boss" monster. Use psychic slime on the first round, then immediately spend your action point to dominate one of the dazed PCs. The overseer is quite a bit tougher than the slime mage, so don't be afraid to mix it up in melee and daze enemies with your own tentacles before dominating them. Finally, the overseer's invisibility power makes it easier to get into the water or otherwise reposition in a losing battle.
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