I just finished a battle with summoned ghosts and got a screen saying they will join my army if I perform
a necromantic ritual. It says I need to cast a ritual spell in the strategic map mode.

There is a button just under the ghost icon but it isn't glowing so nothing happens when I click on it.
Any ideas.

Thanks.

Either:
1. You don't have slot for Tier 3 unit
2. You don't have enough crystals for this ritual
3..You don't have high enough Necromancy to subdue him and window appeared for lower tier undead, summoned in this battle.
4. Bug.

I think you answered my question. I don't have a slot available for tier 3. Thanks.