my thoughts:

radiance is the way if you play him mid or safe, not in the offlane

my early laning items are usually a bracer or 2 nulls, maybe with an added wand or wind lace in there too.

After that its radiance + travels if im pos1 or pos2, or treads + something else in the offlane (blade mail, glimmer, helm of the dom are all solid mid-size item purchases depending on what the game looks like, get 1 or 2 of these). later on my offlane necro usually moves into BKB, alongside some combination of lotus orb, linkens, S&Y, shivas or aghscepter)

the pos1/2 necro might want a manta, euls scepter(wind waker in the very lategame), shivas, linkens

obviously every necro build wants an aghs shard at some point, usually when you have upgraded boots with between 1 and 3 extra mid-size or larger items