Captain America (1968) #306

"The Summoning!"

As Captain America and Captain Britain find themselves chained in a dungeon, Modred uses the Captain Britain costume to lay waste to London, demanding that Merlin appear to fight him; The Captains succeed in freeing themselves and Cap Britain convinces Modred to give him his costume back so he can summon Merlin; The magician refuses to materialize, however, and the Captains have to take down Modred on their own; Once Modred has fallen, Merlin arrives and takes him into an alternate dimension.