Once you level up Sadist, mana is not a problem while farming. That's not the case in team fights, unless you are lucky enough to get a quick kill or have tons of creeps around.
A level 11 Necrolyte with no items has 611 mana. Death Pulse costs 185, Level 2 Scythe costs 340 mana. Without mana items, you won't even have mana for two pulses + ulti.
A level 11 Necrolyte with just int threads and radiance (because he rushed rad) has 715 mana. That's JUST enough for 2x Pulse + Scythe. Note that said Necrolyte has 834 health and 3.5 armor, and thus crumples like a wet paper bag to focus fire. And you can bet you will be focused with that Radiance.
Threads + Radiance is incidentally about what you can expect to have in an "average" game after 20 minutes unless you're getting fed or have free farm. That's 300GPM, which is slightly more than 5 creep kills per minute.
Now, let's say you use that 6550 gold threads+radiance would cost, and buy Wand (509), Arcanes (1450), Meka (2306), Point Booster (1200) and Vita booster (1100) instead. The level 11 Necrolyte now has 614 mana from items, meaning 1225 mana in total. That's enough for Meka (150), Scythe (340) and four Pulses (740 - needs to regen 5 mana to have enough). This Necrolyte also has 8 more armor and 852 more HP (not counting additional heal from pulses) than the int threads + radiance Necro.
(You could go bracers into drums instead of wand and a booster if you wanted to, for better chasing but slightly lower pools)
Of course, it's all pretty irrelevant if you have a sub-20 minute Radiance without forsaking all other items. If you can pull that off, you're so far ahead that it doesn't really matter if it's the right item - you're winning anyway.