Speaking as a powergamer, this combo does not excite me. It's there, it works, it's nothing special. I'd much rather be a Mobile Dire Wolf from variant human.
Is it the most powerful combo in the game? No. But here is my beef: when I ran the last low level campaign with a Druid, battles almost always ended the same way. Nearly everyone would be near 0 hit points and badly in need of healing. Also all their spell slots would be used. The Druid would be completely unharmed with none of their spell slots used. He was constantly arguing that battles were super easy. The rest of the party, I think, wanted to kill him.
It wasn't a huge surprise that the Druid was being played by the guy in our group who was constantly bringing up weird rules interactions that I didn't even think of and trying to use them in order to gain huge advantage.
The only time he only slightly began to worry is when all the enemies focused him. When I'd do that, he would just barely lose all his hit points in wild shape and take a couple of points of damage to himself before the battle ended. But most days only involved one combat so he wasn't worried about using his wild shape every combat. There was only one combat because they were mostly traveling and I was rolling one random encounter a day since more than that was getting boring.
This combo just seems like something he'd come up with in order to prevent even the one hit past his wildshape that he took each combat just so he could brag that he took 0 damage all combat. Given the only way to make one encounter a day hard was to use enemies with really powerful single attacks, it would have been especially useful against them.
Plus, it fails my standard "smell test" for power gaming:
1. does this combination seem "intended" by the rules? No, I doubt the designers anticipated this particular combination. When the racial ability was created wildshape didn't work the way it does now. You got temporary hitpoints instead of replacing your own hitpoints. The rules were changed, likely to make it a little easier for some people to understand. With the side effect that now you reached "0 hitpoints" before switching back to humanoid form. It's very likely that the designers made this change without realizing the other effects it would have on the rules. Which is a common cause of rules holes and other opportunity for power gaming.
2. Does it seem to make sense in character? Not really. Half-Orcs are tough because they have big, strong bodies. They are able to withstand one blow more than anyone else before going down. I don't even really think of them as being hurt at all while in wildshape form. It seems more like a magical construct that prevents them from getting hurt given that part of hitpoints are pain and morale and Druids can lose 0 of their own hitpoints while in wildshape. The Druid themselves is perfectly fine when their wildshape reaches 0 hitpoints, so the in game premise seems to fall apart.
3. Are there 2 or more reasonable interpretations of the rule, one of which is significantly more powerful than others? Is the player arguing for the most powerful interpretation? In this case, sure, one interpretation is that YOU aren't reaching 0 hitpoints when your wildshape reaches 0, it is just an arbitrary threshold that your magic ends at and for simplicity it uses the same mechanics as the hitpoint rules. The other interpretation is that it uses the same mechanics so it's exactly the same and therefore can be used. The second interpretation means you can use it to prevent a lot of damage from a big attack, it can decrease the number of resources you need to spend that day since you won't need healing or hit dice because of it. Even if it only prevents 5 damage, that is one healing spell you don't need. If the Relentless Endurance ability is used the normal way, it can't prevent any more than 1 point of damage given that all damage below 0 goes away anyways. The only effect it has normally is to allow you one more round of attacks before you go down. Which, not coincidentally, is the special ability of Orcs in the last couple editions. It has no effect on resources for the day since healing from 0 or from 1 is almost identical.
4. Does it seem more powerful than similar options? Most of the "damage shield" powers in the game have very small numbers. Things that give out temporary hitpoints are often below 5 points, so when compared to the ability to stop 5 points of damage, the ability to stop 40 damage is extremely powerful. Always assume worst case scenarios when comparing power because power gamers will use it in worst case scenarios. Especially when you assume that in normal use it absorbs ONE damage, it does seem way more powerful than similar options. It pales compared to wildshape itself, which is one of the most overpowered abilities in the game, but that's precisely why I'd disallow this...the character already has a huge advantage over the other players, they have wildshape. They don't need any other benefit to put them further ahead.
