You have 12 classes you can select your heroes from! Each of the classes has 6 various leaders, 2 of each rarity: Gold, silver and bronze. The type of team you have will be greatly affected by them and the rarer they are, the much better!

You can get new leaders from Incantation, so I recommend doing so whenever you have actually gathered enough gems. There are 4 kinds of necromancy:

Apprentice Incantation (50 gems): 100% Bronze leader
Knowledgeable Incantation (200 gems): 72% Bronze leader, 24% Silver leader, 4% Gold leader
Specialist Incantation (1000 gems): 75% Silver leader, 25% Gold leader
Master Incantation (2500 gems): 100% Gold leader.

I highly recommend saving gems for the Master Incantation, as that will be the most gratifying of them all, providing you a Gold leader 100%. You can attempt your luck with the other necromancies if you do not desire to wait till you have actually collected 2500 gems nevertheless!