Realms of Arkania: Star Trail

The spell you need to summon ghosts is quite literally called "summon spirit" so it shouldn't be hard to find. You will also need a magic caster with the spell. I had a druid do the job for me, but any mage that can actually cast the spell will work. Preferably with the best skill level you can find for the spell. Finally, you can only summon creatures in combat, so you'll need to engage in a battle.

On your magic caster's turn, click on the character themselves and choose to use a spell. Cast it, and if your skill is high enough, a ghost will appear next to you.

Do this 10 times for the achievement to unlock. You can summon multiple spirits in one battle as well.