For the new folks, The Decisive Battle of Nidhogg is a campaign with two parts, each differing in mechanics. A Nidhogg iteration typically doesn't associate itself with the event it "piggybacks" on.

The first part, Twilight Underlings [link], involves fighting Nidhogg minions, while the second part, Sweltering Heat Maze [link], features the inside of Nidhogg and has sets of tiered stages, up to the Core level. Flower Medals, the Nidhogg-only currency, can be earned from these stages. The links have the mechanics on each part. They're quite extensive enough that I won't have to type everything here. Feel free to ask if you want clarifications though.

Quote Originally Posted by Myers View Post
I kinda have an idea of what's going on, but not entirely. Does the Nidhogg get stronger each day? Each milestone? Or is the Nidhogg I'm fighting the strongest it's going to get?
"Strongest" in this sense pertains to Underlings on the last wave (Wave 3/3), but in terms of day-by-day increase in difficulty? Subjectively speaking, no. In the last Nidhogg iteration (November 2018), I was able to win* with only "1 Turn" (on the turn counter) in majority of the fights, except on days where I struggled with trying to match the attribute(s). The only other, and more notable, change that happens daily (at 00:00 JST) aside from their appearance is change of weaknesses.

*Win condition in Underling battles is defeating all 3 waves in 5 turns or less.

Milestones are just there simply as milestones.

Quote Originally Posted by maotd View Post
It's Nidhogg's ass kicking time ! Yeay! I didn't expected it so soon. That's a good thing. I like Nidhogg events.
It was on the 4th Anniversary Nico Livestream's table of plans after January. Also, it's been over 2 months since the last one, so we were due for this anyway.