Conversation so far:
When did the reign begin?
The kingdom of Gwynedd began with the reconquest of the coast by northern Britons.
Who were the Britons fighting?
Gaelic tribes from Ireland.
How did the conquest go?
Generations later, Maelgwn's father Cadwallon Long-Hand completed the process by destroying the last Irish settlements on Anglesey.
Who started the conquest?
Maelgwn's great-grandfather Cunedda Wledig.
Where did the name Gwynedd come from?
He is thus most commonly referenced by appending the name of the kingdom to his own: Maelgwn Gwynedd.
When did his reign end?
Tradition also holds that he died at nearby Llanrhos, and was buried there.
What was at Llanrhos?
CANNOTANSWER.
What was nearby Llanrhos?
Deganwy, in the Creuddyn peninsula of Rhos.
What was at Deganwy?
By tradition, his llys.
What was his Ilys?
English: royal court, literally hall.

Question: What is he known for during his reign?
Self-contained rewrite: What is Maelgwn Gwynedd known for during his reign?