So, we all know that Ulic-Qel-Droma was renowned for his knowledge regarding the immunity or resistance against the Dark Reaper that was lost with time.
Now, a few of us have assumed that the Dark Reaper feeds on Force energy like Sith Lords: Darth Nihilus and Lord Vitiate and that renders Anakin Skywalker/Darth Vader immune/resistant to the drain of Nihilus; this unfortunately it is not true.
Now, the Dark Reaper has inside it a device called the Force Harvester.
A lot of people assumed that the 'Force' word refers to Force energy; this is not true. In fact it was already established that the Dark Reaper needs Living Force to function(stated by Mace Windu). The Living Force represents the lifeforce; whereas the Cosmic Force represents the Force energy.
In the Revan novel it is clearly mentioned a difference between the two 'forces':
―Revan
Now, let's talk about the Dark Reaper immunity
Now, if you mark Ulic's words; he also says that this knowledge comes with a risk. But why? Well, that's because the immunity to the Dark Reaper comes with the technique of absorbing lifeforce around you.
"(...)to harness the power of the Force."
"(...)knowledge comes with a risk."
Also, Ulic says it is: "(...)for a short time."
Why? Well we know that the Dark Reaper, once activated, it unleashes lightning bolts of Life-Drain. And the more it consumes; its range the greater becomes.
Thus, in order for Anakin to resist the Dark Reaper's effects; he has to drain the lifeforce around him, hence: "(...)for a short time." and "(...)knowledge comes with a risk."
Below you have a depiction of how the Dark Reaper works
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__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."