You mean Myrddin. Who's based on Merlin without actualley being Merlin. You should read Slaine : The Treasures of Britain as the actual King Authur's Merlin features in it.
Anyway, I always had the impression that he let Slaine keep the Cauldron ( That he had already mysterioulsy obtained...) for his own tribe, so that he may become king. He never gave it to him. He noticed that Slaine had it already and decided to let him keep it. Perhaps he has the right to claim magical treasure for what he was. Perhaps he was the original owner.
Altyhough you may still be correct, and that maybe a reason why 'Cauldron of Blood' is hardly ever referrred to.
( Looking through 'Slaine The King')
Fourth page on the top panel. it says in the box narrative.
'As a reward for helping the Ever-Living ones Myrddin had allowed Slaine to take away the legendary Cauldron of Blood....'
Take, not give
Myrddin is saying in that very same panel.
'Take good care of it. It is the most sacred of our treasures
and will be of more use than any weapon.... when you become the Sun King of your tribe.'
To your credit. It is hard to tell what Myrddin means exactly when he says it is the most Sacred of OUR treasures.
Wether it had always been the property of the Ever-living ones. ( The Slane Diceman story does contridict with this, though.)
Wether he now claims it as their own most sacred treasures, now that Slaine has brought it back.
Wether the Ever-living ones had the thing stolen from them by Slough Grunsgul of the Glass Tower in the past and Slaine had merely been sent to retreive it. Perhaps it has Myrddin's inititials Oghamed on it somewhere.
Wether the whole trip to the Glass tower was a totally random event and Slaine had stumbled across this old Atleantean treasure by accident.
Though I do prefer the explanation that 'Cauldron of Blood ' offers. Re-reading through that part of Slaine the King, it certainly fills in the gap where Slaine is just seen to be attaching the Cauldron to the Knucker.
Unless something that I missed happening at the end of the last chapter of 'Time-Killer' 'The Tomb of Grimnismal' ( which was strangley ) left out in the souveneur hard back edition.
The Spoils of Annwn or even 'Tomb of Terror' have something else to add to this.