Basically, there are 5 actual missions which are kinda lame. Between each mission completion you are forced to defend your base in a night raid. I personally went after all the upgrades before anything else just to be safe. On the final mission it really helps to have the breath control skill unlocked. The mountain yetis will force you to run a bit. Destroy the relic, then the roots, then the relic to complete the saga. Overall I wasn't very impressed and the weapon perks don't seem to do much other then keep you playing longer.