I see Relentless Endurance as an interrupt (for lack of a better word) based on this wording: "you can drop to 1 hit point instead." The feature gives you a choice: go to 0 hit points like you were going to or use the feature and go to 1 hit point instead. For me, the wording of the half-orc feature puts the continuation of Wild Shape in limbo until the player decides whether or not to use that feature.
Personally I would probably not let Relentless Endurance apply in this case since you aren't really dropping to 0hp, you are just losing all hp of your wildshape form and revert to normal form, without dropping unconscious. The wildshape form just ends, like a spell expiring.
But it's not a problem to rule that wildshape is more natural than magic, and so an ability to "endure" physical wounds would help you stay longer in wildshape.
It's only the part about
leftover damage which makes me favor the first interpretation (i.e. disallow Relentless Endurance). When you really drop below 0hp you ignore excess damage beyond the part that takes you down to 0 (except that you may need to check for instant death from massive damage, but if that's not the case, then it doesn't matter if damage would take you to -1 or -10, in 5e you just default to 0hp).
However in wildshape the excess damage
does matter, it is not ignored but subtracted from your original form's hp. Relentless Endurance would then also negate
any excess damage.
Example:
- you have 100hp + 1 wildshape hp left
- you take 100hp damage
- normally you lose wildshape and drop to 1hp
- with Relentless Endurance, you keep wildshape and still have 100hp+1hp
It's only 1/day however, so it's not a big deal if you let it happen. Or alternatively, you could rule that Relentless Endurance holds your wildshape but at the same time it drops you to 0hp+1hp, meaning that you drop unconscious as soon as your wildshape ends.