A remarkable EP, but I wouldn't call it their true debut. I know what you're saying in regards to the stylistic changes, but I'd argue their beginnings were their most influential pieces of work. In the Sign of Evil still stands as one of their most well know releases and although Obsessed by Cruelty is often glossed over today it was an important album in its time. Those first two releases served as an inspiration to many of the upcoming black and death metal bands. I do think Sodom reached their musical peak in 1987, but by the time they had fully transitioned to thrash metal it was already a very popular style.
6.13 Nocturnal Blood / Volahn