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

Yes, but it wouldn't do anything

The Archlich can use a legendary action to cast an at will spell. The casting time of the spell is not specified, so the fact that Counterspell requires a reaction does not invalidate it. So yes, the Archlich could definitely cast Counterspell as a legendary action.

But most of the time, it wouldn't do anything.

Counterspell's timing specifies when it must be cast to be effective:

Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell (PHB, p. 228

To successfully stop a spell from being cast, Counterspell must be cast while another creature is casting a spell: not when they have already cast it. And if a creature is currently casting a spell with a casting time of an action, a bonus action, or a reaction, then their turn is not over. Only once their action, bonus action, and/or reaction is completed (and they have finished doing everything else they wanted to do on their turn) have you reached the end of their turn. And since:

Only one legendary action option can be used at a time and only at the end of another creature's turn.

that tells us that by the time the Archlich has an opportunity to cast Counterspell as a Legendary Action, the Counterspell will no longer be effective. (Once a spell has been cast, you want Dispell Magic, not Counterspell, to remove it). So although the Archlich could certainly spend their Legendary Action casting Counterspell, they couldn't actually foil a PC's spell with it (unless they used their Reaction to cast it).

NOTE: There are some rare exceptions. If a PC is casting a spell with a longer casting time (such as a ritual, or a spell with a casting time of 1 minute), the Archlich could Counterspell it with a legendary action. But spells with longer casting times are rarely cast in combat (as they'd take 10+ rounds), which is the only time that Legendary Actions tend to come up. Erik pointed out another possible exception, which is that spellcasting often takes more than one turn to complete when you are under the effects of a Slow spell, so your spellcasting could be counterspelled at the end of your turn if that was the case (but as the archlich in the adventure you mentioned doesn't have access to the Slow spell, that scenario is also unlikely to come up). And since it's an edge case that's likely to come up, I want to point out that a Readied spell can't be countered since it has already been cast (just not released).

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    One exception that might come up is the Slow spell, which gives spells a 50% chance to take 2 turns to cast, and another is when a character takes the Ready action. At least, I think Ready can be countered? Might be worth adding. – Erik Sep 25 at 9:57
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    The Slow spell is a great example, and I'll include it and credit you. But a Readied spell can't be countered since it's already been cast (but hasn't been released yet). – Gandalfmeansme Sep 25 at 13:20

An 'At-Will' spell simply means that the spell can be cast an unlimited number of times.

Counterspell in particular has a casting time of 1 reaction (specifically, 1 reaction, which you take when you see a creature within 60 feet of you casting a spell).

This means that the Archlich you mention can only cast 1 counterspell per round (you only get 1 reaction per round) to effectively stop a spell.

So, even though the Archlich could cast counterspell at-will, casting it as a legendary action is not the same as casting it as a reaction to a spell.

There's also a more in-depth explanation in this answer to the question "Do legendary actions allow multiple reactions prior the next turn?".

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