This Encounter Culture focuses on illusion, with a deadly, often udnerappreciated, monster that is the absolute pinnacle of deception and comes loaded with everything you need for a truly memorable ordeal.
CR 7- 3,200
Enemies: One Aboleth
The Premise: On paper this encounter is just a battle against an Aboleth, but in reality it is a nightmarish struggle to determine what reality really is. The Aboleth hides behind layers of illusions while dominating as many members of the party as possible.
The Breakdown: What should be a simple looking battle unfolds as a terrible waking nightmare. Start by designing a simple looking dungeon room or natural terrain area that seems to fit with the players location. A kitchen, a prison room, a forest glade, a swamp, whatever you like. This is really the first layer of illusion, created by a Aboleth's Mirage Arcana. The real terrain features large pools of water, completely hidden by the magic until a hapless player falls inside. At this Point the player who fell in is subject to the first dominate person attempt of the Aboleth. If they succeed on their save they may notice some kind of effect tickling the back of their mind, but they have bigger issues to deal with. The water is full of monsters! Depending on the location of the encounter it may be sharks, hydras, krakens, piranha, or other creatures. This is a quadruple whammy of deception. Some of these creatures were generated by Programmed image, Set to trigger when a creature enters the water. Others are created through persistant image, and one of them is actually a Projected Image that has been affected by a veil spell to look like a different, non hostile, underwater creature. Meanwhile the real opponent, the actual Aboleth is hidden behind an Illusory wall. The real Aboleth just bombards the weak willed with dominate person and hypnotic pattern until it has no more dominate and only one or no players remain able to act and then it resorts to melee attacks. The real trick here is that because several of the fake enemies are made by one creature and several are made by another only some of them will become transparent on a successful save, and a save is only allowed for those who interact with the creatures. Which means any party member wanting to know the real deal has to get in the water and touch some potential monsters.
Hooks: Depending on the other encounters the Aboleth may be the big boss of a dungeon, or it may have its own separate far reaching goals. If the players never actually discover the creature and its able to dominate party members without being caught it may even choose to let the party go. Commanding the victim to "Act normally" and then keeping telepathic tabs on them until they can use their new puppet to send more helpless victims its way. A clever Aboleth could even set up an alter ego for captured party members, forcing them to act during the night for its goals, and allowing them to remain normal during the day. Just a few powerful pawns are more than enough for an Aboleth to create an empire from.
Variations: If you don't believe in the Aboleth's ability to get the job done on it's own you can up the CR of the encounter to 8 by pairing it with a CR 5 monster, or group of monsters adding up to 5 (2 CR 3's, or a CR 4 and a CR 2 work). This encounter can also easily be changed up by altering the terrain and the situation in which the players run into their foe. Any template that adds new offensive spell like abilities to the Aboleth can also be very effective, such as half fiend adding unholy blight and Poison, as well as darkness for added confusion. This encounter also serves very well mixed with deadly traps above the water that are cloaked in the illusionary battlefield making them extra deadly. Even just adding some pits that drop the players a dangerous distance before hitting the water work nicely.
Tale of the Tape: This encounter is CR 7 as it's really just an Aboleth using all of its abilities appropriately, and the water is a necessity since that is the creatures natural habitat. Keep in mind of course if any of the Variations mentioned above change the CR that you reward the players accordingly. If you want to give the players a better chance you can give them a chance to avoid detection by the Aboleth so that it won't have it's persisted images in place and won't have a dominate person readied when a player falls in right away. You could also tip them off that the area may not be real by making the illusion seem out of place with the surroundings, such as a foggy swamp inside of a stone dungeon.