So I’m looking over the rules for monster advancement and lI want to build a Mimic that is a castle Medium to Colossal (only one room would be it’s mouth clearly) but could use animate objects to manipulate objects with in (as well as several haunts)
Few questions:
1) Do the size bonus stack since it says continue progression cause that would be a +8 per size category if so for example.
2) The table doesn’t clarify the attack bonus change (do these numbers stack) or damage dice like what does 8-10 mean?
3) While I understand the CR would be over 20 what would you rule to allow a lower level group to overcome the challenge besides slaying the mimic? Escape from the haunted living castle? What would be a fair EXP payout. Normally I give half exp for surviving but not overcoming
What you're describing seems more appropriate as an adventure than a single monster.
2nd edition D&D did have gargantuan mimics called house hunters but they aren't actual structures they are more like snails with extremely ornate shells that look like buildings.
For an adventure you would probably just take a normal setting or structure and populate it with a bunch of things like you describe all under the control and/or extensions of the living structure.
If you want to simply upsize a mimic then I would use the house hunter I linked above as a guide.
To answer your rules questions.
1) Yes, the bonuses stack. If a creature goes from medium to large it gets +8 to str, -2 to dex, +4 to con, and +2 natural armor. If you then bump it up from large to huge it would get an additional +8 to str, -2 to dex, +4 to con, and +3 natural armor as per the chart.
2) A creature gets a bonus/penalty to AC, Attack, CMB/CMD, fly, and stealth based on its size. Once you've determined the final size of the creature it will receive bonuses and penalties based on that final size only.
3) As the DM you can adjust things as needed to represent the actual danger the threat represents. While an encounter may involve a creature that is a CR 25, in the context of the adventure perhaps it only represents a CR 7 challenge. This could be because circumstances prevent it from using the full force of its abilities and/or it isn't motivated to kill the PCs. It may prefer some other course of action (such as simply leaving) after a certain amount of resistance. Meaning that the PCs can effectively "defeat" the creature without killing it. Once this "win condition" is met the PCs should get full exp as per the challenge they overcame. Which again may have only been a CR 7 encounter even though you were using a CR 25 monster.
An adventure sounds like a cool alternative with keeping the house alive as its a Mimic. Players get stuck in a horror adventure locked in not understanding the actual house is against them not to mention haunts and maybe a serial killer rogue or a lich Necromancer (uses magic to control the N castle to corruption)