It is a memento for those who successfully complete 758 Soulpit fights. You can use it as decoration. And yes, the item is stored in your store inbox.
After completing the last required Soulpit fight, the following server message is shown: "Only the bravest could vanquish so many souls! Visit Gnomadness in Gnomprona to commemorate this with a keepsake!!"
According to TibiaWiki, repeated soul cores still count as separate fights. There is a video on YouTube by player Mathias Bynens that apparently confirms this, since he got the reward with only 729 animus masteries. I believe he repeated some fights to help friends:
https://www.youtube.com/watch?v=INQ6Mjh8orM
I don't know any other players who have this item, but maybe this player I mentioned has it in his house.
Source:
https://tibia.fandom.com/wiki/Large_Obelisk