* AnticlimaxBoss: Bokrug is, with difference, the easiest Ancient One in the game. His Doom Track is relatively forgiving, and if you're diligent killing monsters and closing gates you can get rid of the four Beings of Ib before he awakens, which leaves him with very little means to screw you over in a significant manner. Additionally, one of his Sinister Plots makes him turn around and go back to sleep, ending the game in ([[AntiClimax an admittedly anticlimactic]]) victory for the Investigators.
** Yig has been statistically one of the easiest to defeat Ancient Ones, even between different version of him.
** He hits hard, but the battle with Ithaqua is basically a straight-up physical brawl.
** Azathoth, despite his RocksFallEveryoneDies combat is still fairly easy to beat as he takes a long time to wake up, has a very minor game affecting ability and sealing gates tends to be the most common way to win. He's often recommended as a beginner boss because he's so simple and people can focus on learning basic strategies.
* DemonicSpiders: The Hound of Tindalos; it's one of the few monsters that can attack you in any location, can move anywhere on the board, and if playing a low sanity character (poor Monterey Jack), it's almost impossible to fight as it will just nuke your brain instantly.
** Plain ol' ghosts from the main game also count. They're immobile, not to mention ''immune to physical attack'' and have hefty penalties on Will, Fight and Sneak checks. In the wrong place against an ill-equipped character, they are a formidable roadblock.
** Dholes. Resistance to both stats, lethal damage, high stats and toughness...the only thing keeping you sane is that it moves normally, meaning you can generally walk around it.
* GameBreaker: Mandy Thompson is undeniably the strongest investigator in the original game and arguably this with expansions included. She starts with 4 clue tokens, making it much easier for her to seal a gate and more importantly, her ability allows one investigator per turn to reroll failed dice in a skill check meaning that she can be the difference between an investigator sealing a gate, being defeated by a monster, passing an event check etc. It's only somewhat mitigated by her rather high stakes personal story and low stats.
* ThatOneBoss: Great Cthulhu himself. His cultists affect your sanity like most other monsters. Everyone permanently loses a point of maximum health and sanity with him in play. In the final battle, he regenerates a doom token after every attack.
** Atlach-Nacha is the most feared Ancient One in the game, and for good reason; sealing gates, the main method of defeating the Great Old Ones, barely inconveniences him at all. He is also extremely difficult to defeat once he awakens (though not impossible like Azathoth).