In one play by forum game ( https://boardgamegeek.com/thread/1641255/descent-campaign-xxv-1v1/page/6 ) there are several opposite opinions on a question whether traps (pit trap, tripwire, web trap, etc.) can be played on special terrain spaces (water, hazard, lava, sludge, etc.) or not .

I want to call for the current community to help us solve this question. Thanks in advance!

Why not?
These cards must be played on empty spaces... the spaces 'type' are not taken into account !

At least it's how I understand it!

Edit: after looking at the BGG thread.
It's not because some terrain spaces have special rules that they 'block mouvement' : mouvement is alawys allowed it has just some more coditions to do it (pay more mouvement points for example).
So they keep been empty spaces.

As long as the space is empty, Web Trap, Pit Trap etc can be played. The type of terrain doesn't matter.

As has been said, there's nothing preventing traps in special terrain. You can play a Pit Trap in a Pit Space, if you want to. Hell, you can play a Pit Trap in Lava if you want to.

In the thread, Morthai cites the most relevant section just when the argument starts (from I can tell). The only special terrain that does not count as an empty space is if it's an Obstacle, since they block movement and line of sight. Water, Lava, Pits, etc; none of it blocks movement or line of sight.

I'll fourth the opinion that special terrain, except Obstacle, counts as an empty space and is valid target for a trap.

I will add that some players may feel that it's unfair that a well-played card causes them to lose all their MP while on a lava space and thus, at the end of their turn they get defeated, because they did not plan for it and left the move for their last action, and maxed out on stamina...but isn't that what the OL cards are all about after all?

"Empty space" has a very specific meaning in D2E. That meaning is not the colloquial "contains nothing," either. Fortunately, the exact relevant section of rules is on display in that thread. According to the base rulebook, for a space to be empty, it must meet 3 criteria (really only two, because the first one listed is redundant with the others):

1) The space must contain no figures .

2) The space must not block LOS (anyone must be able to trace LOS through it)

3) The space must not block movement (anyone must be able to move into the space)

As I said, point 1 is kind of useless, as figures block movement and LOS. Maybe it will matter if there someday exists a figure who does not block movement or LOS.

Note that as stated in that same paragraph , a space does not need to be actually physically "empty" in the general sense of the word to be considered an "empty space", as spaces containing tokens (which do not block movement or LOS) are still considered to be "empty spaces". In exactly the same fashion spaces containing water, sludge, lava, hazard, crumbling, or pit terrain meet the criteria of blocking neither movement nor LOS. Therefore, they're empty spaces.

As mentioned, obstacle terrain is not an empty space (but that's because it explicitly blocks movement and LOS- the nature of obstacle terrain.) Also, there is no "space containing" an elevation line, as elevation lines lie between spaces (therefore spaces adjacent to elevation lines are also empty).

As a final check: if the space is not empty, the hero is not legally able to end his movement there (such as a space containing another figure). This is why the trap can't be played, as traps may force a hero to stop. Since it's not a legal place to end movement, forcing him to stop there is not legal, either. Since it's legal (though unwise) for him to end his movement in lava, you can pit trap him in lava.

Thanks for your reply, guys!

I'll fourth the opinion that special terrain, except Obstacle, counts as an empty space and is valid target for a trap.

I will add that some players may feel that it's unfair that a well-played card causes them to lose all their MP while on a lava space and thus, at the end of their turn they get defeated, because they did not plan for it and left the move for their last action, and maxed out on stamina...but isn't that what the OL cards are all about after all?

I am convinced that the sole purpose of Overlord Cards is to make heroes shout out "Noooo! It's not fair!" .

I live for those moments. Especially if it involves forcing them into lava somehow and then throw a web trap on them. I've only done it once, but the look of the hero when he realized that he's immobilized in lava, forcing him to not only end his turn there, but to play out the entirety of his turn knowing he was going to die. Good times.

I see 2 reasons for OL cards, and both come back to "uncertainty."

1) Like you said- without OL cards, heroes could be nearly certain their turn would turn out like they hope. "I'm just going to move over here, then search". "Actually, you're getting stunned, sorry!" OL cards force them to make backup plans, and keeps strategy interesting.

2) Monsters really only improve between Acts, but heroes are constantly improving with new skills and gear. OL cards give monsters variable abilities so the heroes never know QUITE what a monster is capable of. "I'm out of range of that ettin, even if he takes a full move action to come get me." "He would be, except for this DASH. Incidentially, this attack is getting a DARK MIGHT and a CRITICAL BLOW."

I suppose OL cards do exist to make the game more interesting for the OL, too, but most of the joy comes from those "it's not fair!" moments.

In a more serious vein, the Overlord Cards is really necessary for there to be any kind of dynamic game going on. I must say that I think that the abilities and skills (such as Elder Mok's Heroic Ability, or the Just Reward and By the Book Skills of the Marshal) that messes with the Overlord Cards in one way or another were a really bad design decision. I assume that the idea was to create a way to interact between the heroes and the overlord in a more direct way, rather than through proxies, but ultimately, I think it largely detracts from the game; that element of uncertainty, not knowing what cards the Overlord has or when he's going to play them, the subtle psychological interactions between the heroes and the Overlord (and between the heroes as they interact in opposition to the Overlord) is really at the core of the game.

We've all had moments of "Should we do that or not? What if he has Pit Trap?" and "OF COURSE HE HAS TRIPWIRE, LOOK AT HIS FACE!" , and without Overlord Cards, without them being secret and without them being random, that would never happen. I might as well play chess. Maybe that's why so many people find the Lieutenant Cards so uninspiring and unnecessary - they're small decks, you can only pick from that one deck, they've got a rather clear progression, they're all always in play, and once revealed, the heroes know exactly what you have anyway.

Overlord Cards, though? Even with just Basic or Basic II, the heroes will never know what the Overlord has, and the Overlord will (almost) never know what he'll get. The long hours I've waited for Tripwire to come around, I swear...