TIL The Cursed Library is quite do-able with a 7/8 maxed Priest, although the Tome of the Tarnished Gods probably has a lot to do with that. I had also figured out while working through 1500 fame on Archer and Wizard that Manor of the Immortals is tough, but pretty consistently do-able. Still haven't mastered Haunted Cemetery yet; I may need to work on my Summoner cheese to beat boss Skuld. Rogue is supposedly another good class for that dungeon.
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A few days ago I was mulling over plans for my D+D character, Tuzo (Rng5/Rog1/Ftr4). After Fighter 5 I figured I'd circle back to Rogue for Assassin subclass and ever more evasion and Sneak Attack. But Assassin seems like, boring? because this character is silly optimized already. So I thought Soulknife would be a cool alternative subclass, but I reviewed a few other subclasses just to make sure I understood the options.
Fairly often in the D+D subreddits, people post along the lines of "what would you do if your DM said you could take multiple subclasses of a single class?" For skilljacks, the answer must be Phantom Rogue's "Whispers of the Dead" (lets you pick up a bonus temporary skill or tool proficiency each day) crossed with Soulknife Rogue's "Psi-Bolstered Knack" (may add a d6 anytime you fail a proficient skill check and costs no resources if the check still fails!).
But on consulting Reddit I learned you don't even need the double-subclass situation (which no sane DM would allow). MMotM's Githyanki race gets a feature even better than "Whispers of the Dead" (gain proficiency in [one skill] AND [one weapon or tool]). And Ravenloft's Reborn race gets "Knowledge from a Past Life", usable [Proficiency modifiers times per day], ergo a weaker version of "Psi-Bolstered Knack" (usable 2x[Prof.mod] times per day).
Because Soulknife doesn't spend the usage if the roll doesn't change the fail to a success, and their Psionics dice scale up at higher levels, the clearly-preferable configuration to combine these is the Githyanki Soulknife. I mean, a Reborn Phantom Rogue would be very thematic too but the former combo squeezes more cheese.
Adventurer's League allows only one accessory source per character, so no combination of Soulknife or Phantom (both source: Tasha's Cauldron) would be "AL-legal" on a Ravenloft or Mordenkainen's Monsters race.