I’m a DM in a very homebrewed campaign and one of my players, a necromancy wizard who serves Fate, wants to make a homebrew spell. In and of itself that’s not an issue, we’ve made homebrew spells before, I’m all for that. 
He’s wanting to make a spell that is essentially the Rage Beyond Death feature that Zealot barbarians get at level 14. Other than making it level 7 (the spell level you get at level 14) I’m not sure how to balance it. Any tips? 
Here is the Zealot feature for reference

Beginning at 14th level, the divine power that fuels your rage allows you to shrug off fatal blows.

While you're raging, having 0 hit points doesn’t knock you unconscious. You still must make death saving throws, and you suffer the normal effects of taking damage while at 0 hit points. However, if you would die due to failing death saving throws, you don’t die until your rage ends, and you die then only if you still have 0 hit points.

*EDIT*
How would concentration checks be affected through this? My original thought is to force concentration checks, but the player has the thought of since you don’t *actually* take damage you wouldn’t be subject to them.

Well, I would make the duration of the spell Concentration 1 minute, and make the range touch. And since most characters don’t have the Rage feature, all references to Rage would have to be removed and changed to represent “the duration,” kinda like this:

Endure Beyond Death

LEVEL

7th

CASTING TIME

1 Action

RANGE/AREA

Touch

COMPONENTS

V, S, M *

DURATION

Concentration 1 Minute

SCHOOL

Abjuration

ATTACK/SAVE

None

DAMAGE/EFFECT

Buff

 

Until the spell ends, having 0 hit points doesn’t knock the target unconscious. It still must make death saving throws (as normal), and suffers the normal effects of taking damage while at 0 hit points. However, if the target would die due to failing death saving throws, it doesn’t die until this spell ends, and only if it still has 0 hit points at that time.

* - (a shield, and diamond dust worth 100 gp, which the spell consumes)

Something along those lines at least.

This is very helpful! Very similar to the lines I was thinking, but one block we hit was concentration checks, the player doesn’t think he’d need to make them if it was cast onto himself. 

IMO of course they would. They still get hit and take the damage, just because they can’t go any lower than 0 HP shouldn’t matter.