Blue orbs (or spheres) can be obtained during gameplay or by spending real money. You need it in order to speed up the rituals and upgrades and also to buy exclusive things. 

There are several ways to obtain blue orbs:

1. Complete missions
Tap on the thief icon on the right side of your dungeon to open missions menu and find out what missions you have. Completing missions gives you orbs and experience. When you complete a mission, a new one pops up in its place.

2. Level up
You receive orbs with each new level. To level up, collect green points of experience scattered all over the levels, both single player and multiplayer.

3. Win leagues and guild battles
Finish in Top 3 of your league or be one of the top contributors in guild battle to receive orbs.

4. Lock picking
You will sometimes earn a few orbs while trying to open a dungeon's door.

5. Visit the Store
Tap on the "+" icon near orbs to buy more with real money. You can find out how to disable in-app purchases in the Billing section.