To what extent is a legal will obligatory, and what are its provisions or most important pillars?

The original ruling for a will is that it is commendable (mustahabb), though it can become obligatory (wajib) or forbidden (haram). Its essential component (rukn) is the declaration by the testator, which can be any expression indicating ownership transferred after death without compensation. Examples include: "I bequeath such-and-such to so-and-so after my death."