What happens when you cast Death Ward on a half-orc? Do the effects of the spell and the half-orc's Relentless Endurance racial trait stack?
If so, and you get reduced to 0 HP more than once, can you go back up to 1 HP a second time? Or does only one of them take effect and you can only be brought back to 1 HP once?
My DM thinks the latter.
Death Ward will trigger the first time you drop to 0 hit points, and Relentless Endurance won't. Then next time you drop to 0, Relentless Endurance will be ready to go.
Death Ward, emphasis mine:
The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends.
As soon as you would be reduced to 0 HP, you are instead reduced to 1. As opposed to the Relentless Endurance trait, emphasis mine:
When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.
You choose when to use it. It doesn't automatically trigger when you are reduced to 0 HP. So when you are reduced to 0, with both Relentless Endurance and Death Ward, just don't use Relentless Endurance. Death Ward will be used, then next time you drop to 0, you can use Relentless Endurance.
There's another point to be made here, though. Even if you want to use Relentless Endurance, while you have Death Ward on, you can't.
The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends.
Death Ward triggers when you would drop to 0, and makes you drop to 1 instead. You never actually drop to 0.
When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.
Since Relentless Endurance is triggered when your hit points are reduced to 0, and Death Ward prevents you from ever reaching 0, you couldn't use Relentless Endurance even if you wanted to. This argument also works if your DM believes you don't get to choose when to use Relentless Endurance, which you asking this question suggests he does.
