There is no save to make. It's that check after taking that action. If people in the campaign are spending actions constantly investigating everything as they travel they're going to travel rather slowly, lol.From elsewhere, players can disbelieve from a distance. Take a Study action to make an Intelligence Investigation check versus the spell DC. This is in addition to any interaction or save.
We need to ask ourselves why and how someone might investigate from a distance, or how they interact. Hallucinatory terrain isn't anything more than another terrain feature for characters to see until after they take that action when there's no reason to take that action in the first place.
It's also a large area, long duration, and unlike most illusions doesn't require concentration.
Most people go around that stuff if they can to avoid it. I have yet to see a fighter find a tar pit and say "I gotta get me into some of that" instead of going safely around it.Many players would normally interact with tar pit, quicksand, or bog. Not sure how Hallucinatory Terrain would make a bog seem "poisonous". A wildfire would suspiciously lack heat.
A bog that replicates the smell of rotten eggs would register as poisonous and potentially dangerous to someone familiar with the concept.
The DM should be describing the illusion as if it's real within the senses of the players. So if there's a smell that's dangerous because toxic fumes exist within gassy swamps then the DM might call for a survival check regarding the smell instead of acting like the smell isn't real to that player at that point in time. The character obviously smells the smell.
It's because the spell isn't worthless. It's niche in it's practical application because it's essentially a tool for trickery in a restricted environment.Heh, I am unsure why anyone would try defend a worthless spell, but I guess this is the internet. Maybe its just the intellectual challenge of coming up with any utility whatsoever. But even if, the infrequency of such a situationality would make the spell worthless.
A bard or warlock being limited in spell preparation aren't likely to take it outside of conceptual reasons. Or at least I would advise against it because of the niche aspect. A wizard who finds it and adds it to their spell book isn't hurting themselves in doing so and might use the spell occasionally now that they can swap it in on a short rest. A druid might prep the spell on a day when they plan on using it.
The illusion looks, sounds, and smells real so the DM should be treating it that way until the players do something to change that.
Mirage arcane is similar but it's range is huge, it's area is huge, it's duration is long, it also doesn't require concentration, and it does add that tactile component that came up.
Remember, there isn't a saving throw for these spells. They're just terrain features until such time as the players specifically take an action to investigate them. A person needs to be careful with the tactile aspects of hallucinatory terrain but not mirage arcane.