rangerofthewest
The Destroyer
1) It's not related to rainso, after more 10 hours or so i managed to drop it, killed about 700 red devils total. I went to the surface by the time it dropped and guess what, IT WAS RAINING, which makes me think: Maybe the drop is related to rain? since it dropped the first time it was raining?
2) There is nothing wrong with Terraria's RNG
3) You just have bad luck
Sources: I am a programmer.