PLEASE stop comparing 
and DFG.
Deathfire Grasp primarily scaled with your own stats. Duskblade ONLY scales with the enemies stats. (their missing health)
This means they have absolutely NOTHING in common. Duskblade isn't even an active. If you want a comparison, I would compare DFG and Ghostblade. Those are better comparisons.
But please, stop comparing these items when nothing about them is even remotely similar.
Stars Shaper4 votesActually DFG made the enemy take magic damage in % of their max hp.
Dealth1 votesYou are right I fixed it.
Zero Skill Tank3 votesThe point that was similar was magic damage amplification, not initial damage on active, and this was the most problematic part of DFG.
If you dished out X damage during amplification period, DFG would increase it by percentage. If you dish out X damage before duskblade procs, it will deal percentage of it again, effectively increasing it by percentage. That is assuming target was full HP at start, if it was not it will deal even more damage.
The only thing that makes DoD better than DFG is that you can not control it that easily, so you can't decide 'yo, this guy is tanky I better mark him for more damage' as you could with DFG.
Paroe2 votesYoure right. DFG was VISIBLE damage.
DBoD is invisible damage.
DFG also scaled with your enemies stats, columbo. It was a 20% max health pop.
Dealth1 votesSure. I forgot about the max health damage. It's a lot less than 20% though and the invisible/visible thing you said makes no sense actually it's quite the opposite.
Jefftiffy1 votesSure. I forgot about the max health damage. It's a lot less than 20% though and the invisible/visible thing you said makes no sense actually it's quite the opposite.
Also this item does more damage than DFG does in most cases. And building an item that punishes you for building specific stats is usually a bad thing. It would be like making an item that does 1.5 extra physical damage for each point of MR you have.
Paroe1 votesSure. I forgot about the max health damage. It's a lot less than 20% though and the invisible/visible thing you said makes no sense actually it's quite the opposite.
DFG: Actively shoots a projectile which deals 20% maxHP damage. you KNOW when its DFG that kills you. DBoD: Passively does 25%missingHP damage. you DONT notice when its DBoD that kills you.
Dealth1 votesDuskblade puts a big mark over your head then it explodes I think it's fairly obvious. The first part of DFG is visible but the second part (what people bought it for) is definitely not.
Meep Man1 votesDuskblade actually does scale with your damage, since the more damage you do the more missing health they will have. Not quite as powerful as what DFG did, but it does.
Mecha MaIphite1 votesDuskblade ONLY scales with the enemies stats. (their missing health)
Damage amp and %missing health damage is very similar. Damage amp will amplify the damage you do to a target in a time frame by a certain percentage, %missing health will amplify the damage that champion has taken up until that point. Or in a solo lane, the damage that you have done.
It sounds like %missing health is much stronger because it factors in all the damage from before rather than just for a short time period, and it is, but just remember that you could control when you used DFG, but Duskblade is a different story.