[MENTION=38357]kaomera[/MENTION] So your problem is challenging your PCs right?
Quickleaf: That's an interesting encounter, and I may have to try something like that at some point... The issues I have would be: Using MM2 monsters, as I'd rather stick to MM3 or MV stuf at this point.
Sure that's your perogative. In play I've been fine doubling static damage of pre-MM3 monsters, and reducing three defenses of elites by 2 (solos are another matter). Why wouldn't that work for you?
I think I understand how the Derghodemon is supposed to work, but I'd be worried about a level 12 (or even level 13) monster being able to consistently hit the PCs. Especially since it looks like it needs two hits in one turn to start a grab.
Whoa, you have 13th level PCs right? The derghodemon should be hitting an average AC of 31 (+17 +2 CA with prone from earth furrow +2 flanking with xorn). Do most of your party have ACs over 30?
As for the death hook, I'd expect one PC to be grabbed every other round (not to mention if there is another grab the derghodemon is sustaining). And once every 3 rounds I'd expect a PC to get successfully death hooked. That seems reasonable.
This is an even bigger problem with the Xorn. And in general I wouldn't expect much from the minions, to the point that they seem a bit like useless complexity.
Low-level minions aren't about piling on damage or genuinely threatening PCs - they are useful in that they help the more powerful monsters simply by their presence.
I choose Xorn because they (a) fit the flavor of the encounter, (b) can burrow up in waves from unexpected directions, (c) their multiple attacks mean it's likely with PCs who are close together that at least one is taking a little damage. Since the %chance to hit is low but they make multiple attacks (making %chance of criticals higher) I would give the minion-ized Xorn extra damage or a rider effect on criticals. And I like minions which cause an effect simply by being adjacent to a PC in numbers, such as the Hanged Ones, so I would replace Submerge with some kind of "if a PC starts their turn with 3+ Xorn adjacent to them..." (eg. They are grabbed)
Also consider how the minions interact with the rest of the encounter. For example they have darkvision in a potentially unlit cavern and they can burrow anywhere around the cavern without the PCs knowing where they'll come from next, this may grant them greater manueverability during fracturing fissure.
You might decide there is something spawning extra Xorn each round until the PCs destroy it. Or you might decide to give the minion-ized xorn a feature which enhances the derghodemon's grab attacks/sustaining grabs.
I'm sorry, I kind of just panned everything in that encounter, I guess...
Yeah you did

but I may not be articulating the synergies of this encounter as well as I'd like to - the individual elements are probably easy for your PCs, but it's how they work together that makes this a challenging encounter for your living fortress party.
It demands mobility from the PCs on multiple fronts (do i go after perched artillery? do I save my skin to avoid a hazard or do I stand by ally's side to fend off attackers no matter what? do I go after the Xorn generating device or focus on taking out the derghodemon? do I dive single-handedly into the derghodemon's burrow where it dragged my friend?). It also demands the ability to adapt to sudden changes (Xorn popping up, Stormstone fury delayed explosive thunderstones, triggered hazards).
Your party is a tanking living fortress. Divide them, require mobility, provide changing/evolving challenges, and in general don't fight fair (focus fire, send lurkers after squishies, etc).
I just feel like I've been trying to add more monsters, synergistic stuff, interesting terrain, etc. and it seems to just make the encounters longer, not more fun.
It sounds like you are trying a lot!
Is it a game prep/info tracking issue, or party's composition issue?
In the second case, its clear the players want to be tough as steel and nigh invincible so let them play to their strengths. You get to exploit their weaknesses
Here's a quick list from what I've gathered:
* Encounters can last too long, so any threat which grows in intensity each round is going to challenge a party focuses on staying power over hard-hitting
* The party shrugs off damage and effects...so look at putting more traps/hazards & controllers into your encounters since they change the game the most
* The players don't seem to care about fights, taking success for granted...so throw in alternate combat objectives besides "kill all monsters" (TheWeem has a great blog about this).
* The swordmage's aegis has been used to devastating effect in your games...so consider encounters where line of sight and being within range can't be taken for granted. Teleportation in general requires the DM think differently about encounters.
Where are those hazards from? I've been looking at including some hazards, but all of the ones I can find seem very odd & impractical...
The cave-in is from the DMG, the fissure I made up. Apologies, I tend to assume other DMs are as comfortable homebrewing as I am.