When Mercenary Inc. catches up to Rita on Floor 25, she uses the Focus Crystal to control the Dargon and commands it to fight Mercenary Inc..
After Mercenary Inc. defeats the Dargon, the Focus Crystal loses the power to control the Dargon (since the Dargon no longer exists), so Rita gives Mercenary Inc. the Focus Crystal. She also mentions that in addition to being able to control the Dargon, the crystal can also be used to open up the lower levels of Evil Cave.
During the credits, Koohii is shown saying that she thinks that a crystal would be a good thing to steal. That likely implied that she might attempt to steal the Focus Crystal, however no such action is taken.
After the credits, Tristam check with the shop to see how much the Focus Crystal is worth, but since it couldn't be used to summon the Dargon it had no value. However, Kaeli finds out from her sister that there is more treasure in the Evil Cave, so Mercenary Inc. uses the Focus Crystal to access the lower floors of the Evil Cave.
- While Mercenary Inc. can obtain the Focus Crystal, it does not show up in the inventory. This is likely due to the fact that it is only used to access the lower level of Evil Cave.