Sorry if this is already asked and answered somewhere - search capability on this forum could use some work :>

The way I read the expansions is you add in all cards from the expansion that don't require treachery, then (if you like) spend threat to swap out cards with cards that require treachery. However, if you're playing with several expansions, you're adding quite a few cards to the overlord deck, increasing its overall size, and thus also increasing the length of time required before you reach the end of the deck and are able to steal those yummy 3 conquest tokens. The effect is as follows - 3 cards from Well of Darkness, 6 from Altar of Despair and 4 from Tomb of Ice, for a total increase of 13, or over 1/3 of the original deck size.

Increasing the deck size has two negative impacts on the overlord: 1) Slows the time period for reaching the end of the deck (by up to 6 more turns for the current set of expansions, even more over time); and 2) Dilutes the deck, increasing the time needed to find that "one critical card" the overlord might be counting on to make the hero's lives miserable.

Was this negative impact on the overlord of using expansions the intention? I.e. Using expansions is felt to add some benefit to the overlord through increased variability of the nastiness you can employ, and the heros are therefore cut a break by the impact on the overlord of an increased deck size?

The alternative would be to say there's a fixed limit to the deck size, or at least a "minimum" deck size of the standard deck (36 cards). When beginning the game, the overlord chooses which "standard" cards they want in their deck, ensuring the size is exactly (or at least) 36 cards before swapping any of their cards for treachery cards.

My personal preference would be to go with a minimum size deck rather than a fixed sized deck. If you want to pump your deck to 40 or 49 cards with all the expansions, you can, but if you want to run with a trim deck of the base 36 cards, incorporating your favorite expansion nasties and throwing out standard cards you don't like, then that's ok too.

Is there an official ruling anywhere? Does anyone else play with a trimmed deck, or does everyone just throw in all of the non-treachery overlord cards and live with the longer time to end-of deck?

Almost universally it seems people just add the extra cards in.

In general, the power level of the added cards is higher than the average power level of the 'originals', so although the deck gets larger, it also gets stronger. It does seem to even out overall.

In vanilla Descent (ie non RtL) the much higher proportion of spawns is quite an improvement for the OL.

In RtL there is a cheap OL upgrade that allows him to remove 4 cards from the deck. Since massed spawns are not so useful in RtL (15 extra threat for the second and subsequent spawns in each level, and smaller levels), often these are used to remove some of teh least useful spawns.

The addition of Treachery is also very powerful for the OL. Note that Treacehry cards replace other cards, so the rubbish gets tossed for great cards, further strengthening the deck.

IIRC someone tried trimming their deck back down to 36 cards and found it overly powerful?

Yeah, adding the expansions gives the OL more options but more cards to go thru. It only adds a few cards per expansion. It's not a big deal, and the bigger variety and ability to swap out junk cards for awesome cards sufficiently compensates. I'd bet the OL is stronger as a result of the expansions, since you can ditch all your worthless cards that you never use an replace them with awesome ones that do more damage than the standard deck cards.

If you got to add them by swapping out old ones, your deck would become insanely strong and lean, and that's not fair.