When the Lion King appeared in the Sixth Grand Order, she summoned the Heroic Spirits of the Round Table with her own power.
(This is different than the summoning method used by Chaldea)
All but two of the Knights responded to the summons, and were informed of the world's collapse, and how in half a year's time all would be reduced to nothing by the King of Magic. And, the Lion King told them about the "means of salvation by the holy lance," their best hope of counteracting it.

Camelot/Zero