Is there a list of movies to watch before this?
Well there are fifteen in the DC Animated Movie Universe:
1. Justice League: Flashpoint Paradox
2. Justice League: War
3. Son of Batman
4. Justice League: Throne of Atlantis
5. Batman vs. Robin
6. Batman: Bad Blood
7. Justice League vs. Teen Titans
8. Justice League Dark
9. Teen Titans: The Judas Contract
10. Suicide Squad: Hell to Pay
11. The Death of Superman
12. Reign of the Superman
13. Batman: Hush
14. Wonder Woman: Bloodlines
Really, you should watch all of them before this one, but if you're looking for a minimum, the ones I've highlighted in bold. And at the very
very least, Flashpoint, War and Dark. Then this one.
Flashpoint sets the timeline, War introduces Darkseid with the first clash, the Batman ones introduce Batman's son, Teen Titans ones develop the Raven angle, Suicide Squad introduces Harley and her motley crew, and the Superman ones re-introduce Darkseid's scheming behind the scenes.
Probably the most disposable ones are Hush and Bloodlines, although they happen to be very good.
Apple have a few of these in 4K (in the UK we didn't even get BD releases for all, let alone the 4K that the US got).