The Sorceror's Stones
After assisting Arthur in Royal Rampage, speak with Merlin inside the castle.
Follow your compass to find the stones. They are- behind some barrels in the castle,
by the English billboard,
on a Nueva York rooftop,
amongst undergrowth by a tree near the pig jousting field,
on a dome-top in Wakanda,
in the bushes by a panther statue in the Wakanda palace complex,
in the corner by a ramp in outer Wakanda,
atop that quart-circle building in Manhattan,
on a ruined building ledge by the bay in Manhattan,
and in the coreward-most corner of Liberty Island.
Return all ten to the castle to unlock Merlin.
This also unlocks a Gwenpool mission in the castle.