I don't know if this is a bug or an intentional design. Anyway, on the "War for Survival" map, there are succubi near a suspension bridge, and if you attack them with a secondary hero and one creature of level 1, they will join you. I've checked it 5 times, and they always join.
P.S.: This is good news for Necropolis and Inferno players.

This is a terrible bug in the map that simply prevents you from holding tournament duels on it... The succubi only join the red player, and they always join, regardless of whether you attack them with the main hero or a secondary hero. The blue player cannot recruit them.

And as a bonus, the recruitment is not only available to Inferno and Necropolis, but also to Stronghold, who can get about 7K gold for them, and Dungeon, who also have the opportunity to sacrifice them.