In Appendix A(ii) to The Lord of the Rings, Valandur's place in the list of Arnor's Kings is marked with a dagger (†) indicating a premature death, but without further explanation. This typically implies a death in battle, but this does not seem to apply to Valandur; Arnor was involved in no war that we know of at this time, and at the age of one hundred and ninety, it seems rather implausible that the King would have gone to battle in any case.
It might even be tempting to dismiss this single dagger character as some kind of misprint, except that it also appears in the more detailed lists of Kings in volume 12 of History of Middle-earth. Unfortunately, in Valandur's case that list is only marginally more detailed, and we're given no more than the perfunctory note, '†slain'.
Perhaps we can take Valandur's death as a hint of the divisions brewing in Arnor that would lead to its dismemberment some two centuries later. If so, it had no obvious direct result at that time: Valandur was directly succeeded by his son Elendur, who ruled for the next 125 years. However he met his end, Valandur's mysterious demise is a reminder that there was more going on behind the scenes of Arnor's history than we see in the chronologies.