To sum up the confusing comments and semi-answer:
The two words eventuell and Ereignis are NOT (semantically) related.
Das Ereignis is a noun and can be translated as the event.
eventuell is an adverb and can be translated as maybe, perhaps, possible, etc.
Eventuell gehe ich heute Abend zum Fußball spielen. -> Maybe I will go play soccer tonight.
Ich weiß nicht, ob dieses Ereignis eintreten wird. -> I don't know if this event will happen.
I don't speak Portugese and I don't know which translator or dictionary you used to look up these two words, but the first online translator I found translates evento correctly into Ereignis. Translating eventuell yields por acaso or possivelmente.