The invoke command is tied explicity to objects. An object can seem normal in every way except that it may have a special capability which can be activated in differing locations. The ability is hidden, even from the divinations that mages may make and only by taking the object to the correct place and then invoking it can you activate the ability. More often than not the act of invoking an object will drain it completely and it will be destroyed.

Examples :

: go temple
Great Temple
You stand in front of a polished marble altar.

Obvious exits : out
:invoke sceptre
As you call on the power of the sceptre a ray of light from the altar
strikes first it, then you. A burning sensation behind your eyes quickly
fades away and when you look up you see a ghostly portal seemingly floating
in mid air.

:look
Great Temple
You stand in front of a polished marble altar.

Obvious exits : out, portal
: