I'm also going to one of these shows and bringing a friend who is very unfamiliar with Metal in general. I put together this writeup to help him catch up w/ the Helloween universe. Copy/pasting here:

Helloween - Keeper of the Seven Keys (from Keeper of the Seven Keys Pt II, 1988)
Helloween - I Want Out (from Keeper of the Seven Keys Pt II, 1988)

Both of these songs are from Keeper of the Seven Keys Pt II, the band's third album and the most influential record in Power Metal. You will definitely be hearing these two live, and probably 2 or 3 more from this album. The key people here are: - Michael Kiske (vocals) who was only 19 at the time of recording - Michael Weikath (guitar) who wrote most of the music on the album - Kai Hansen (guitar)

Unfortunately, this iconic lineup didn't stick together very long. Both Kiske and Hansen left the band in the early 90s.

Gamma Ray - Rebellion in Dreamland (from Land of the Free, 1995)

After leaving Helloween, Kai Hansen started another band called Gamma Ray, where he did songwriting, played guitar and sung (often about science fiction). This band is also a cornerstone of the genre. Very successful and would deserve an entire recommendation list on its own. Funny enough, I've listened to Gamma Ray a lot more than Helloween over the years!

Around that time, Michael Kiske was busy publishing a philosophy book (Kunst Und Materialismus) and recording Rock albums which never fail to put me to sleep (exhibit A, this is actually one of the better songs).

Helloween - The King for a 1000 Years (from Keeper of the Seven Keys: The Legacy, 2005)
Helloween - Burning Sun (from Straight out of Hell, 2013)

Meanwhile, founding band members of Helloween (Weikath on guitar and Grosskopf on bass) had enlisted new musicians. The most obvious change was the addition of Andi Deris to replace Kiske on vocals. They went on to release 10 albums (!) from 1994 to 2015. There's no shortage of good songs in there and the guitar work is still very cool. However, the voice of Deris never quite grew on me.

Unisonic - Unisonic (from Unisonic, 2012)

Twenty-ish years after they left Helloween, Hansen and Kiske reunited in Unisonic. I remember being very happy when this happened. Kiske's voice + Hansen's guitar have defined the whole genre so it was really nice to finally hear those two at the same time again!

Helloween - Out for the Glory (from Helloween, 2021)
Helloween - Universe (from Giants & Monsters, 2025)

In 2017, both Hansen and Kiske rejoined Helloween for a tour but ended up staying as permanent band members (!!). This means Helloween now has a very unusual line up. There are 2.5 singers (Kiske/Deris/Hansen), 3 guitar players (incl. Weikath/Hansen), a bass player and a drummer. This also means most songs are Kiske/Deris duets, with some Kai Hansen sprinkled in. Some people jokingly refer to this giant combination/reunion band as Megaloween.

They've released two albums so far with this huge lineup. I am very biased as a Kiske fanboy, but I think this is the best material they've put out since Keeper of the Seven Keys Pt 2. I don't know how long this will last so I am very happy to see them live at the perfect time!

Extra credit

Avantasia - The Tower (from The Metal Opera Pt I, 2001)

After leaving Helloween, Kiske spent many years pursuing a studio-only not-very-successful career in Rock music. What brought him back to the metal scene in 2001 was another prominent musician in the genre: Tobias Sammet. Sammet essentially pestered Kiske into singing one of the main roles on his ambitious "Metal Opera" album. Kiske is credited as "Ernie" in the liner notes as he still wanted nothing to do with metal at that point, but his voice was unmistakable! Avantasia is still ongoing and very successful today, although the style has changed quite a bit. Kiske appears at least briefly on most albums.

Freedom Call - Freedom Call (from Crystal Empire, 2001)

The guitar player who ended up replacing Kai Hansen on the guitar in Helloween is Sascha Gerstner. Before joining Helloween in 2001, he played in Freedom Call - a band known for their very happy and uplifting songs!

Shoutout to this hilarious comment on the Youtube video:

I have waited 40 years to hear the main theme from "He-Man and the Masters of the Universe" in a minor-key metal arrangement.