The emphatic nūn attaches to certain forms of the imperfect verb and imperative. Its heavy form is doubled (with shaddah), while the light form is vowelless. It can strengthen assertions or commands in verbal constructions.
لَيَحْضُرَنَّ زَيْدٌ
Zayd will certainly attend.
The emphatic lām and heavy nūn reinforce the prediction. Inna and emphatic nūn do not occupy the same grammatical position: inna typically governs a nominal clause, whereas emphatic nūn attaches to a verb.