All That Glitters...

This vase is likely the urn of King Terenas Menethil II. In our timeline, Arthas disposed of the ashes before using it to store the remains of Kel'thuzad before he was resurrected. Seemingly in this alternate timeline, some part of Arthas urged him to keep the now empty urn, even after becoming the Lich King.

After turning in the quest to Soridormi, the item is placed among other temporal relics you recover. Clicking on it opens a dialouge box with the following text:

<You found this golden vase at the top of
Icecrown in Azmourne.

Looking at its outside, yo can tell it used
to be intricately decorated.

Time, or something else, as long since
marred it with scrapes, scratches and
gouges along its surface, so you can't
make them out.>

<The urn is empty and its top is missing.

Its inside is soot blackened.

For such a poorly treated item, it was an
odd find at the top of Icecrown, a place
of prominence for the Lich King>

Weird that the more compassionate Arthas who still has some semblance of a soul is the one who succeeded in wiping out Azeroth with the Scourge.