A Nasik (= pandiagonal & pantriagonal) 18x18x18 magic cube is not possible. You can get a diagonal ór a pantriagonal 18x18x18 magic cube.

Diagonal

[Composite 3'] Using a diagonal 6x6x6 magic cube and a (simple) 3x3x3 magic cube (and its inverse), you can construct a diagonal 18x18x18 magic cube.

Pantriagonal

[Method of Luyendijk] the method for double odd I have constructed a pantriagonal 18x18x18 magic cube.

[Composite 3'] Using a pantriagonal 6x6x6 magic cube and a (simple) 3x3x3 magic cube (and its inverse) you can construct a pantriagonal 18x18x18 magic cube.