Is a "clear path to the target" needed for non-spell effects?

Question

The PHB (p.205) specifies the general rule that a "clear path to the target" is required to cast a spell. The rules on cover (p.196) tell us that total cover prevents an attack or a spell.

Is there a general rule that prevents targeting by an effect that is neither a spell nor an attack?

Inspired by Does Meld Into Stone prevent Intellect Devourer attacks? but the question applies more broadly. For example, the Charm action of a Vampire (MM p.297), the Petrifying Gaze of a Medusa (p.214), the Dreadful Glare of a Mummy (p.228), some types of Spores of the Myconid (p.232) noting that the Rapport Spores expressly says it can go around corners but the others do not, the Rotting Gaze of a Nothic (p.236), and so on.

I note that there seems to be a strong theme that these are all gaze-type attacks but I think that's just that these are the most common things that fall into the category and I didn't do a full survey. The Myconid's isn't a gaze attack and I am sure there are others.

Answer 1

There is no such general, explicit rule

The only mentions of a "clear path" in the general rules is the one you already cite. There is no general rule that to target something, an effect requires a clear path to the target.

Apart from that, you have the cover, range (p. 147, 195 PHB) and reach (p. 195) rules that limit what you can target (and a special one for ranged attacks Underwater, p. 198), and the range (p. 202) and targeting and area of effect (p. 204) rules for spells. The DMG adds optional rules about what you can target when you use facing (p. 252).

There is not even a clear statement that non-magical effects behave as we can expect them to, which would help with commonsense applications for targeting questions. The best we get is "the DM describes the environment".

Effects that require line of sight must state so explicitly. All of the gaze-related monsters (Basilisk, Medusa, Catoblepas, Beholder, Chain Devil, Sea Hag, Jackalwere, Lich, Mummy, Nothic, Revenant, Vampire, Scarecrow, etc.) do so, and creatures with fear effects typically do, too.

Many others that have effects that require line of effect tend to use a combination of the statement that they can see the target and a range, for example the Gibbering Mouther's spittle or gibbering, Hobgoblin Captains leadership, the Intellect Devourers devour intellect, Myconid's spores, the Rust Monsters antennae, the Unicorn's shimmering shield or the Will-'o-Wisps consume life. Spells that require you to see the target likewise explicitly say so.