The german distributor of Descent added a leaflet with optional rules to his version. I just wanted to introduce them to the community if these options are not well known already.
I have tested quite a few of them, but two of these give the heroes the heebee-jeebees.
Optional Rule 1 Hidden Maps
Map tiles are not placed when the heroes enter a dungeon level. The heroes obviously should not get a look at the map in the quest guide during the game. Only once heroes actually gain line of sight (LOS) to a map tile is it placed (obviously the starting area is completely placed at the beginning). Also place any obstacles only when the heroes get LOS to them.
Once a tile is placed it remains in game to the end of the dungeon (or level).
In an RtL campaign I construed the starting area to be the area marked in red where no monsters can be placed by the OL. Also I never told the other players the name of the level they were on currently. This kept them in the dark for some time as many levels start similarly.
Optional rule 2 Hidden Monsters
Monsters are not placed on the board when the heroes enter the dungeon but only when they gain LOS to them. Any monsters which are on the board remain visible as long as they are alive. This also applies to spawned monsters as they obviously can not be spawned within LOS.
This gives the OL an immense tactical boost especially at later stages where his monsters actually become a lot better than plain vanilla monsters. Also the heroes dont know how to best place themselves to prevent that mage from being killed from behind unless they go very carefully. Careful advancement again takes time - and time is the OL's friend.
Although these two optional rules sound quite tame they are killers. It removes a lot of the instant tactical knowledge the heroes get from the game. This is replaced with finding the monsters first, thus wasting time and resources. Also it removes the instant dungeon clearing effect a bit as the heroes have to choose their actions a lot more carefully. Guard actions become more interesting as the players don't know if a melee monster has enough movement to really reach them if it came from around a corner just up ahead or if they should wait for its big buddy (which might be too far back, etc.).
If you enforce one of the two at silver campaign level, and both of them at gold you give the heroes a lot tougher time. At least this helped me a lot.
I added my own optional rules to this.
Additional map tiles:
At silver campaign level the OL could add up to 4 small to medium (no more than 2*4 straights or a crossroads, turn-off, or a curve tile) map tiles anywhere on the map except between the starting area (red marked spaces) and the first room. All the original map tiles still had to still be used.
At gold campaign level the OL could add up to 6 small to medium (no more than 4*4 rooms or the above tiles) map tiles anywhere (again placed as above).
Additional obstacles
At silver campaign the OL is allowed to play up to 4 additional 1*1 space obstacles, at gold up to 6 additional 1*1 space obstacles anywhere on the map (OL choice). These hadd to picked at the start of a dungeon level, and placed next to the playing area. Each time a new tile was placed the OL has the chance to pick any or all of the obstacles and place them under the restrictions shown below.
Obviously any of these obstacles have to be placed to leave a legal path to the end of the dungeon for all heroes.
Hidden monster powers:
All monster powers are hidden until the first time their effect is used against the heroes or affects the actions of the heroes somehow. Until that occurs the monster powers (speed, armor, wounds, attack dice, specials) are kept secret. For this keep the monster stat cards hidden during a level until the heroes actually beat a monster at least once. Only those effects the heroes had any chance to notice are immediately revealed (for example pierce, ironskin, etc.). Obviously the real pros already know all the powers, but until then you might keep some players in the dark a bit longer.
All dungeon cards are revealed at the end of a level as well as the map in the quest guide. The hero players may check for any incongruencies with the laid out map and the played monsters afterwards.
Together with the hidden maps rule above this totally changed my later stages of the game. The players grew to be a lot more careful and started using their heads a lot more.
To balance any OL cheating which might occur due to the hidden monster/monster power rule the hero players had the following options: if they could flawlessly prove the OL's cheating they may remove the offending monster/obstacle/tile plus any one other monster or obstacle from the current dungeon level. Also they gained one immediate roll on the chest table plus one conquest token for detecting this cheating attempt. This is normally enough to keep the OL on his toes and play it straight.