_{Image Source | Warframe.com}

Nightwave is a revamp of the Alerts system in Warframe. They added a Lore into it, the first ever Nightwave is for Wolf of Saturn Six.

_{Image Source | Warframe.com}

Instead of time-limited Alerts consisting of 1 mission and 1 reward, Nightwave will be a Tier-based and Token-based system where you will need to complete tasks to rank up your tier, and get items as rewards for leveling up - this includes tokens called Wolf Credits which you can also use to purchase items offered in this system.

_{Image Source | Warframe.com}

Watch here for more info:

I've completed some tasks under Nightwave which placed me on Tier 1 and got me a sygil as a reward. I still have to do tons of missions and tasks before I can rake up Tier and Creds to get the things that I need from the rewards.