Although the preseason patch was only just released, one of the many changes that it brought has already been changed by Riot. This was a change to Dark Harvest which simply made it the single best keystone in the entire game and was subsequently run on almost every single champion in the game.
The change that Dark Harvest received during the preseason was that it would proc on champions that were below 50% HP instead of first needing a soul to activate it. Furthermore, Dark Harvest will now proc on both abilities and auto-attacks, instead of only on auto-attacks. This would have probably been enough to make it a strong rune but it had some serious damage numbers as well. 50-80 base damage and also 8 extra base damage per soul collected. While in late game this is not too significant, this early-game damage bonus was absurd.
It was not too weird seeing a team run Dark Harvest 5 times, as long as the champions they picked synergized with the keystone, which is not too difficult anymore. The burst that you could now do in the early to mid-game was simply too high. Late game it would be manageable but most of the time it was already too late at that point.
Fortunately, Riot quickly realized that the keystone was severely overpowered. They’ve probably also seen some stats on how many times it was chosen over other keystones which is why they’ve nerfed its early game damage a bit as well as the scaling that it receives from collecting souls. Now it will only do 40 damage in the early game instead of 50, while still scaling up to 80 base damage in the late game. Furthermore, a soul will now only increase its base damage by 5 instead of 8. Dark Harvest will now be a bit more manageable in the early game although it will fall off even harder during the late game considering that the scaling of the keystone has gone down.
I expect to still see quite a lot of champions running Dark Harvest as its early game power allows you to win trades very favorable. Winning lane is League of Legends is quite important for winning the game, especially in lower elo, so I expect this rune to still be a very strong option for a lot of champions that want to win lane early.