Only in 1283, at a meeting of the Danehof in the city of Vordingborg, did the Danish magnates force King Eric to install Valdemar as duke.
However, the military campaign proved disadvantageous for the duke, and as he intended to travel north to gain support in Sweden and Norway, he was captured by King Eric and confined at Søborg Castle in 1285.
Just half a year later, in November 1286, King Eric was assassinated, and Duke Valdemar naturally fell under suspicion.
The Queen Dowager, Agnes of Brandenburg, apparently shared the suspicion, but feared open aggression with the Duke.
In 1297, a final settlement was concluded where Duke Valdemar among other things abandoned his claims to the disputed island of Als.