The change to Darkness within addresses one of the biggest issues with Gul’dan’s build viability: his Q build’s mana (and therefore health) thirstiness.
See, Gul’dan’s Q is really strong due to its 4.5% damage scaling. However, its low cooldown means that dealing its damage consistently to optimize its impact can severely tax Gul’dan’s health reserves.
Because he had few talents to affect his ability to balance his mana and health more effectively, it was usually safer to pick the more one-and-done Corruption playstyle, which often had just as much impact without the issue of it severely hampering Gul’dan’s survivability. This is why his E build was so astronomically more popular than his Q build.
I suggested about a month or 2 ago on another thread about Gul’dan that Fel Flame’s build would actually be worth picking if they added a talent that would reduce its mana/health-thirstiness, like something that refunds mana on hero hits or the like. This new version of Darkness Within effectively does just that by removing the health cost of Life Tap after hitting 2-3 heroes with Fel Flame, greatly increasing Gul’dan’s ability to keep up his sustained damage.
The only problem I have with this change is that it should be at a lower level, preferably level 7 (though accordingly weaker). At level 16, it has to compete with one of Gul’dan’s normal Q build talents, Rampant Hellfire.
Considering that the main purpose of this change is to allow Gul’dan to get more use out of his Fel Flame talents, forcing players to choose between Darkness Within and Rampant Hellfire is counterproductive. I know I’d certainly never take RH over DW if they were at the same level, because +25% damage on all my abilities almost permanently at no cost to myself is way better than +50% damage on a single ability that will burn through my own mana and health too quickly to get any comparable sustained value out of.
At level 7, DW would not overlap with any other talents in his Q build, removing this issue entirely. DW would just need to be nerfed slightly to bring it in line with other talents at this level. Hell, they could pretty much get rid of the spell damage bonus entirely and it’d still be a big improvement to his current Q build.