Undead Secret Skill

The Undead Secret Skill provides a chance for any units killed on a normal stage to be revived instantly. This reduces the risk of being pushed back, but later in the game this can become an inconvenience as there are situations where being pushed back quickly is vital.

It can be turned off in the Options under the Setting Cog in the top right.

Stats
Each level improves the strength of the skill and requires less of the corresponding race's units to be in your team.

Related Information
A screenshot of the skill at max level is below.