What are all the rune bonuses

To make sure that they stay dead, you need to behead them.

At least that's what the IGN Wiki tells me:

Death in the Nemesis System

Death is not the end in the Nemesis System: Not for Talion, and not always for the Uruks either. Whenever you kill a Captain or Warchief in Sauron's Army, there is a chance that they will survive the apparent killing blow. This can only happen if the Uruk in question was not beheaded in the finishing blow. Any Uruks that are lying down dead in the army page may come back and hunt Talion for revenge. After encountering them in the wild, they will reappear in the army page, now on their feet and sporting new scars based on their previous encounter. Some Uruks may even gain Strengths and Weaknesses depending on what happened to them.

Captains can get promoted to Warchief when doing "Duel" events.
Here is what a Duel icon looks like and the description of one.

Although the best way to get a new warchief is to accomplish Betrayal missions.

Betrayal Mission

Even at the top ranks, there is always struggle for power. A Warchief’s bodyguard might betray him and damage or outright kill him. If the initial assassination attempt is unsuccessful, the betrayer will have to fight the Warchief and all his other men which would certainly mean his death.

If players choose to step in at this point and kill the Warchief, the betrayer immediately becomes the new Warchief and gains a power level. This is useful for drawing out a Warchief with the plan on killing him especially if one of his bodyguards is already dominated by the players.

(Credits for Betrayal mission)

The new warchief

The new warchief will be one of the captains serving the one that just died.