Now, let's look at a list of CR 13 enemies:
Beholder - quite probably rapes you with eye rays unless you kill it quickly. The beholder doesn't really care about being stuck. He just sits in the middle saying "Thank you for blocking the charge lanes" while using his AMF on someone that looks casty while tossing out save or dies.
He doesn't have line of effect, so he'd have to use the antimagic cone just to supress things and thus get out... but the cone doesn't hit himself and only supresses the overall effect, so he still takes damage and can't do anything but move out. Not the best at taking him out, but handy for messing up his day.
Ghaele - Greater teleport at will.
Glabrezu - Greater teleport at will.
Right, these guys you don't use that on.
Iron Golem - Immune to poison, so you'd have to kill it with ranged attacks. DR 15/Adamantine may or may not be a problem.
Factotum. DR is a non issue at level 11. Iajutsu focus works fine on them, or ranged attacks, or Silent Image (they're not intelligent at all, so just putting an illusionary wall of Obdurium around them solves the encounter). So, Golems are a non threat, but Cloudkill won't work on them.
12 Headed Cyrohydra - You're not going to kill it via regular damage because it's fast healing is more than you do in damage. 4 con damage from stinking cloud won't slow it down much either.
Cloudkill takes it out after a few rounds. Remember, Cloudkill does 1d4 damage per round for a minute per level. It moves away from you so it's better to do this in such a way that the cloudkill hits a wall and stops while still effecting the baddie, but this will definitely drop the hydra with no chance of survival.
Ice Devil - greater teleport at will
Lich: 11th level human wizard - immune to poison and can teleport.
Mummy Lord - Immune to poison and is a tenth level cleric.
These too are obviously not too worried, and you wouldn't use Cloudkill against them (though the Lich and Mummy are just begging for a Turn Undead, which you have access to, especially if you've invested in a Lyre of the Restful Soul and Rod of Defiance... the two of which are cheaper than a single Truedeath Crystal).
Death Slaad - Gets word of chaos at CL 15
Which doesn't save it at all. Once you fire off your opening round Cloudkill/Solid Fog, you don't stick around close enough to get hit by spells! So yeah, it's screwed.
Any dragon - 150' fly speed and flyby attack (using the breath weapon)
The combo was Solid Fog/Cloudkill. 150' fly speed won't save it at all, and in fact it can't fly in a solid fog at all. Dragon goes down (Shivering Touch is obviously a better dragon killer, but Cloudkill/Solid Fog is a more generic attack, so if you weren't prepped for a dragon it's a good way to do it.
As you can see, the combo isn't actually very effective at the level you get it at. A 13th level rogue would of course be trying to flank most of these creatures to death with his fat sneak attack dice.
Actually, about half of what you described would be absolutely hosed. With no ability to move and no ability to see outside of the cloud, you just can't do anything unless you've got some easy escape (teleport) or are immune to poison (no damage, but Solid Fog would still take you and anyone near you out of the fight for a while). You also forgot to mention PC classed enemies (something I've seen plenty of in games), most of which are pretty darn hosed (remember you can't Shadow Teleport if you don't have line of effect, which you don't have in a cloud). Wizards and such might be just fine if they have the right spells prepped, or screwed if they don't. And of course there's things like powerful skeletons and zombies and other templatey things, some of which are hosed, some are not (the skeletons are still screwed by Solid Fog, allowing your party to take out half the encounter before fighting the other half... you'd just not cast Cloudkill).
The mummy and golem own the rogue in core, but with complete adventurer in play, he just uses swift wands to melee the shit out of those too.
Yeah, those wands don't work at all. Not even a bit. First off, they're standard actions unless you're using the Rules Compendium, and even then it's debateable (since the Rules Compendium isn't clear as to whether it's trying to completely overwrite the DMG section, or just restate part of it). Second, they use up a hand to play with unless you're spending a bunch on that. Plus the difficulty in making sure you have them... Heck, I've done this rant a few times, but the point is you'd need Truedeath Crystals for that and those cost a heck of a lot. Meanwhile, let's face it, Factotum against Mummy or Lich or Golem is a downright silly fight with the Factotum having Turn Undead or the ability to just ignore/confuse the golem.
No, the combo of Cloudkill and Solid fog is very effective, and hits a bunch of enemies at once while simultaneously killing them and acting as crowd control. It's not a gaurenteed every time killfest, but it gets the job solidly in a way Rogues just don't do, which is the point. There's a lot of stuff it works great on.
As I've shown above, it's really not that effective at level 13 for your "Once per day mega trick". Oh, and our mega trick used 5 of your 6 IP for that encounter too.
It's not your only trick, but as you actually showed above it's extremely effective on more than half the enemies you might be dealing with, and is especially good at knocking out groups of enemies (like 4 CR 12s or something). It also can be shot off in the surprise round before they get a chance to know what's happening, and that's quite nice. By comparison, the Dragons move too fast for Rogues to deal with effectively, the Golems and Undead create all sorts of problems for Rogues, the Hydra can TKO a Rogue so fast it's VERY unwise for a Rogue to get within 30' of them, the Slaad is another one that can really screw the Rogue, and so on. And more to the point, it's something every Factotum can do past a certain point regardless of gear or feats or whatever. And yet I notice you're still assuming the Rogue has access to things that bypass the undead immunity, which is decidedly not gaurenteed to be true for every Rogue.
I'm by no means saying that Factotums are the ultimate mega killers of all in their path... just that they do what Rogues do far better and with far fewer problems, and are very effective, especially at stuff Rogues have trouble with. After all, a Factotum can spend a lot less on a Rod of Defiance and a Lyre of the Restful Soul than a Rogue has to spend on a Truedeath Crystal, and while the crystal just lets the Rogue not suck completely against undead, the Rod and Lyre let the Factotum one shot a whole encounter of undead pretty much whenever they want. This is because Factotums don't have to spend their wealth just making up for their own flaws, so when they spend a lot of wealth buying or making items to deal with specific enemies, they become actually good (not just serviceable) against those enemies.
And will you stop changing names? Or did you get banned again and are bypassing it again?
JaronK