I want to create a logo based on norse symbols, probably something from here:
but I'm not sure about licence. I want to make minor changes to the symbol and use it as a logo for an event, will it be considered plagiarism?
There are two creators whose copyright you have to consider here:
The person who conceived the symbol itself. They are probably dead since hundreds of years and thus their copyright, trademarks, and whatever have long expired.
The person who draw that diagram in your question. Their copyright is almost certainly not expired, but if you redraw whatever symbol you like from scratch, it does not apply here.