An Archlich in a WotC adventure is listed as being able to cast one of their at-will spells as a Legendary Action. Here's the quote:
At-Will Spell. [the lich] casts one of his at-will spells.
The lich's legendary action is described as:
Only one legendary action option can be used at a time and only at the end of another creature's turn.
Counterspell is listed as one of the spells of the archlich that can be taken "at will."
Given the limitation of the legendary action occurring "only at the end of another creature's turn" – does that mean that counterspell cannot be used as a Legendary Action to counter a spell cast by another creature or PC? If the archlist can use Counterspell as a Legendary Action, does that mean the archlich could also use its Reaction to cast Counterspell – thus enabling two Counterspells in one turn?
The adventure from which this is taken is:
Tomb of Annihilation p.209