I love Kataklysm because unlike most of death metal bands these guys can deliver some really awesome melodies sometimes. Groove and heaviness are a matter-of-course in this scene, but melody is a rare thing and you have to appreciate it. “Blood In Heaven” is just one of many examples, just listen to the chorus! The song itself is very real and emotional, I simply love it very much.
It's played with the standard 5-string tuning (B, E, A, D, G) and I can't say it's super difficult to play, but all those tempo changes can be challenging.