At the end of Act III you'll conquer Minas Morgul.
From here you'll need to either introduce your own captains from lootboxes or dominate ones from the current pool in the city.
Then the final three Pitt Fight missions open up.
Complete all three and you'll be rewarded with a new skill, "raise dead".
You'll then need to purchase the "revive captain" addon to it witha skill point.
There are two ways the achievement will unlock.
One is using the skill, which isn't too hard in the Shadow Wars questline to achieve.
The other is through Army Menu, which costs ingame currency.
1) The skill requires you to resurect a defending captain or bodyguard. You can't simply call your bodyguard and kill him yourself, as that'll result in betrayal and block the achievement.
You can do this easily in the Shadow Wars questline. Let a captain bleed out when they go down and then resurect them in combat. Do this by having a fill might metre and then hitting
together. To resurect them and any grunts around you. Enemy grunts will also be resurected.
2) When a captain dies, you can go to the menu and press
on their corpse, which seems to cost ingame currency.
I prefer using the skill as it turns the tide of battle, but if you didn't get the chance too, the second option is available to you.