More Darius nerfs. The nerf you gave him will probably do nothing because "dealing the killing blow with his ultimate" is how all players play. So, what's the point?
I say completely remove the refresh timer. It seems very uncomfortable and very overpowered. Take Garen for example, who seems to be an earlier "Darius" version. His ultimate deals damage based on enemy's HP, which is fine, but it has no cooldown refresher whatsoever so you can honestly say it's a good skill, you just need to master it more.
Darius, however, can spam it each time he deals a killing blow. That's in the good play, a penta kill, or in the bad play, a double kill at least.