Mdthbs

The plot in my story revolves around a certain ability of the MC that is normally not available, unless the circumstances are just right. Therefore, it is not mentioned in the story as a solution to the MC's problem, as the ability is considered to be something from a legend and not really an option.
I want to avoid to just throw the existence of said ability into the climax of the story as a deus ex machina. However, it also makes no sense to talk about it extensively beforehand due to above reasons. I'm also afraid that talking too much about it will give away the climax to the reader.

How do I foreshadow the existence of this ability effectively without telling too much or too little? Will an anecdote or a story 'from legend' be what I'm looking for? When do I talk about it?

You needn't reveal that the character has the ability, but you need to reveal the fact that the ability exists. Otherwise, indeed, this is a Deus ex Machina.

How you reveal the existence of the ability is up to you. Maybe someone recounts a legend. Maybe it's part of a history lesson. Maybe it is even specifically mentioned to "no longer exist" or maybe you even explain why it's extremely rare and couldn't possibly happen within the story. You could mention the thing once in passing, or you can mention it multiple times as part of your worldbuilding.

Look at some examples: in Game of Thrones, G.R.R. Martin says multiple times that dragons no longer exist, and the eggs Daenerys holds have long turned to stone. By the end of the book, the dragons hatch. In Susanna Clarke's Jonathan Strange & Mr. Norrel, the Raven King is subject of old legend. Then he shows up.

One approach to doing this is to not foreshadow the thing itself, but rather mentioning something similar to it. So if there's legendary powers in the story, one solution would be to mention not the exact power your character is going to have but rather a different power in a different context.

Another idea would be the following: If the special ability of your main character activates under certain circumstances, let them get into similar circumstances and nearly, but not fully activate the power. Maybe they got a tickling sensation when being presented with a certain situation, maybe they felt a rush of power, something like this.

Another (though very limited approach) is to include things like legends as part of religion. Let the character take part in a religious ceremony where the ability is mentioned by the priest, buried below tons of other mystical things that don't actually come into life later. As priests often talk about things that seem fantastical at first glance, you can hide an ability without making it too obvious to the reader.

As Galastel says, you have to talk about it.

I do that with legends; and make sure my hero believes those legends maybe were real, but probably exaggerated, and certainly the descriptions have probably been embellished a bit.

Like believing a religious icon from the past maybe had some miraculous powers, but raising somebody from the dead was exaggerated. Maybe the ancient folk thought somebody was dead but they weren't, so the icon cured him and brought him around, but didn't reverse an actual death.

Or maybe the hero believes absolutely, but also believes it is a one-time thing, anybody claiming to have the power or have seen the power is either a fraud or a sucker.

I tend to include both believers and skeptics, fewer believers and the mainstream skeptics, with logical reasons for being a skeptic and just "belief" for the believers, they don't have any logical justifications.

But then of course in the moment of truth the hero gets slapped upside the head with their ability. I like that surprise and the emotional conflict it causes.