Just an addition to Tomitain's post, the gacha is split into two parts. They have not provided us with a release date on the second one, but we can assume it will be part 2 of next event and will take over RG Gacha schedule from now on as all Gacha 5* have RG now.
If you have noticed, we actually got rework for 10 rainbows, but only half the gold and 5 rainbows are spotlight and obtainable by ticket. You can expect to select the remaining units from the next gacha.

On the topic of Nidhogg, it's disappointing how they have not made much changes to accommodate for modern standards... could at least ask whoever made tower to fix it lol...
Just a demonstration of 4* Nidhogg being ripped apart by Non-Gacha 6* (They're not ampy'd either) + Lily RG without the need of any fancy FM.
https://streamable.com/5jp9mr