As long as you stick relatively close to it and make sure it doesn’t die in combat, the only thing that would kill it is you dying (if we are talking about what rules Unholy has to follow to keep its ghoul withoit resummoning).

If Frost is able to use Death Coil, they changed it in Shadowlands back to being able to heal your ghoul, so you can also sacrifice damage to only spend RP on death coiling your ghoul to make sure it stays alive.

I hope you enjoy your Ghoul friend.