Complex sentence: Although Sirius assumed he was killed by Voldemort or Death Eaters he was actually drowned by inferi after drinking the potion that protected Slythern's locket in an attempt to weaken Voldemeort by stealing it and ording Krecher to destroy it.
Split into simpler sentences: Although Sirius assumed he was killed by Voldemort or Death Eaters. However, he was drowned by inferi after drinking the potion that protected Slythern's locket in an attempt to weaken Voldemeort by stealing it and ording Krecher to destroy it.