There isn't a plural form of it. I can't imagine a situation when it would ever come up.
English grammar note: 'Futhark' is an example of an uncountable noun. Other uncountable nouns include
Futhark is the name of an alphabet. Chess is the name of a game. Neither word has a plural.
Scrabbles anyone?
elder futhark is the oldest known language of the early rse settlers known as the vikings.
As the names indicate, the Eldger Futhark is older than the younger. The former's use is dated from the 2nd to 8th centuries AD; the latter dates from the 8th century AD.
Bridges is the plural form of bridge.
"Groceries" is the plural form of "grocery."
There is no plural form for the word, countries. This word itself is a plural.
"Beliefs" does not have a plural form, as it is already plural. Beliefs is the plural form of belief.
The plural form of "meter" is "meters."
It has no plural form.
The plural form for the noun sergeant is sergeants; the plural possessive form is sergeants'.
The plural form of rung is rungs.
The plural form of mouth is mouths. The plural form of month is months. The Mounth is a range of hills in Scotland and does not have a plural form.
The plural of "she" is "they", so the plural of "she had" is "they had".