The Mishnah in Moed Katan 3:3 says:
וְאֵלּוּ כּוֹתְבִין בַּמּוֹעֵד ... דְּיָתֵיקֵי
They may write the following documents during the festival: ... wills [of a dying person]
The parenthetical addition of "of a dying person" is added in multiple translations so I assume it comes from somewhere.
Who are we able to write a will for? Anyone? Anyone who is dying? Anyone who is likely to die on Hol Hamoed?