Bendigeidfran's head was one of the Three Fortunate concealments (emphasis mine):
'Shame on my beard,' said he 'if I don't open the door and find out whether it is true what is said about it. [So] he opened the door, and looked out to Cornwall and over Aber Henvelen. And when he looked, suddenly everything they had ever lost - loved ones and companions, and all the bad things that had ever happened to them; and most of all the loss of their king - became as clear as if it had been rushing in towards them. And from that moment, they were not able to rest unless they were making for London with the head. However long they were on the road, they came to London, and they buried the head in The White Hill.
And that was one of the Three Fortunate concealments when it was buried, and one of the Three Unfortunate Disclosures when it was unearthed: since no affliction would ever came to this Island from across the sea, as long as the head was in that concealment. That is what this tradition says. Their adventure 'The men who set out to Ireland', is [the name of] that [tale].
Source: The Mabinogi of Branwen
What were the other two Fortunate Concealments? What was their significance, and in what stories do they appear?