*partly, as others have said, the double 1 is so rare 1/36 so assuming 8-10 rooms per quest (at least for most of the earlier ones) so once per 3-4 quests, you can basically discount it. But even if you increased the odds to 1/6 for example ~once per Quest, the variation in strengths of WM in prewritten quests, from Goblins to Chaos Warriors means that a few unlucky rolls could drop an extra 3-4 CWs into the mix that dramatically changes the difficulty of the game. Maybe it would work for homebrew quests if the WM strength was kept low.
At the end of each of his turns, Morcar rolls a combat die. If the result is a ‘white shield’, he may place a wandering monster anywhere on the board, in any room or corridor that the players have visited but no longer occupy. If such an area does not exist, Morcar does not place a wandering monster.
Regards the Games International comments, I think the ‘hit point’ option was covered off by NA monsters stats which I use despite generally sticking with SE rules, but I too like that suggestion (although the use of ‘white shield’ gives a once every 3 turns so my earlier ‘strength of WM’ comment is highly applicable, imagine this with a CW WM) and am certainly going to consider introducing this but perhaps fiddle with the odds, 1/6 feels right from gut instinct alone, but I will certainly be reviewing the WMs for pre-written Quests before I implement this and sticking with lower level Goblin/Skeleton, Orc/Zombie WMs although not necessarily just single ones, but I don’t really see this as a fix for the door blocking issue*, rather a rule to discourage time-wasting, perhaps not as fully featured / rich as a Event/Dungeon/EW Deck but a lot simpler to implement.
*Although as a sort of side effect it may on occasion help with the door blocking issue
Also, in terms of placing the new WM, I agree that 'anywhere that has already been explored' is too broad and allows Morcar to build up a reserve force near the exit for example and other such exploits and may cause Heroes to keep needing to back-track to deal with spawning monsters, personally I prefer WMs to be placed on the first square that the heroes can see (otherwise how would they know it was even there and why would it be placed on the board at all) from any one of the various approaches to the heroes. This better represents them moving (you could even say 'wandering' from a position out of sight of the heroes to a position in sight of the heroes (and therefore being revealed/placed on the board)
For me the key methods for mitigating the door blocking issue are:
1. Diagonal & Ranged Monsters – as a simple minimum tweak I have all Goblins armed with Spears as standard and Skeleton’s Scythes allow diagonal attacks, and you can expand this to include other less common monsters, like the Gargoyle, and the occasional use of ranged weapons (and I’ve also added a couple of extra Goblins into a few Quests as this change makes them more useful and they are under-represented in the base game)
2. Push back rule (2 variants) – which is discussed at length here: Houserule: Push Back
= white skull, one "hit"
= black skull, one "hit"
= shield, cancels out one "hit"
