Hi folks!

Epic game with my son last night. He was amped up to 11, and all he needed was a Talisman. He got himself a Warlock's Quest, which I believe was 'deliver 3 gold'. I had the Desolation spell (turn a location into either a Ruins or a Desert).

Now we weren't playing the Firelands - none of those cards were in the Adventure deck, but I had for some reason left the expansion's spells in the Spell deck (we like to add only the expansion(s) we're actually playing).

So, if I had cast Desolation on the Warlock's Cave, would it then have become impossible for him to fulfill his quest and get that Talisman?

I believe there is no official answer.

The current consensus seems to be that you would be required to complete the Warlock's Quest and the Portal of Power is locked until you do, yet you neither teleport to the Warlock's Cave nor receive a reward.

This hasn't happened to me, a mistake or not, we would probably discard that spell and draw another depending on usage and timing and all.

Remember: you can have a Talisman in a lot of ways, for example by the Temple or in the Adventure deck
Abuout the Terrain card: The Warlock Cave is the only critical space because the quests and rewards. In the Rules there are examples with Sentinel, Temple and Portal of Power...
Can I suggest you some House rules?
1- Ignore your warlock quest: you can enter in the inner region. If you complete the quest nothing happen

2- The Warlock Cave is like the Jail and the Town square of the City expansion: Terrain card cannot be placed
3- The Warlock Cave is gone... but the Warlock is still alive: you can teleport in the ex-warlock cave only if you complete a quest for a reward, you cannot have new warlock's quest by the warlock cave.

conclusion from here:

https://community.fantasyflightgames.com/topic/100694-terrain-cards-warlock-quests/

So, the rules included in the base game rulebook are actually rules about Warlock Quests, not about the Warlock's Cave space. Well, in the base game there was no difference between the 2 concepts, but now the picture has changed quite a bit. I think it's safe to assume that if the Warlock's Cave is Terrain-ed, you must still complete quests as soon as possible and the Portal of Power remains sealed; you just don't get teleported to the Warlock's Cave and don't receive a reward for your quest.

If we treat the rulebook words and the Warlock's Cave words separately, the effects would be this:

1. Gain a quest. (Space)

2. Must complete the quest as soon as possible. (Rulebook)

3. Cannot cross Portal of Power with incomplete quest. (Rulebook)

4. Teleport to Warlock's Cave once quest complete. (Space)

Thus the Space effects are gone, but not the rules which target the Quest and not the Space, as they are still in effect.

So in conclusion if the Warlock space is Terrained, then Quests must still be fulfilled but because the space is gone there is no reward. However it is fulfilled and one can now enter but they have to get a Talisman elsewhere! But no more new quests from the warlock, he doesn't exist!