scribendus

Etymology
Substituting the missing present passive participle (gerundive) of.

Participle

 * 1) which is being written

Declension
Necessity usually pertains when the gerundive comes with a conjugated form of esse, e.g.:
 * 1) Epistula scribenda. - "The letter (which is) being written."
 * 2) Epistula scribenda est. - "The letter has to be written."