The 5H mixup:
5H-5D, you gotta be pretty damn close, otherwise the 5D will whiff. 5H-5D only has a 5f gap inbetween. 5f is most character's fastest attack, usually their jab. So if they miss their punish by 1f then it trades in your favor. Outside of RRC blockstring mixups, to land a 5D you have to heavily condition your opponent to block low and not hit buttons. Incidentally, 5H chains into 2H and 2D, which are both low hits. 5H-2H, if you have them in the corner, will not push you back. (Unless they FD, FDing this string is about -30% tension) Which is important for how close you have to be to connect with what could have been a 5H-5D. 5H-2D can be used to setup a frametrap and discourage jabbing/reinforce low block.
Escalation:
5H-2H is -4 on block. Punishable by throw, if their head is on right, that is what they'll do. But their head maybe a little loose, 'cause they should of thrown you when you got close enough to start the 5H mixup. If they press buttons on you, then it's the usual mixups off disadvantage. If they are so scared they do nothing, then you can rerun the 5H mixup on them.
5H-2D. From the range that you would have to do 5H from, 2D is crazy punishable, and it'll be punished hard. Either take the punish, or RRC. If RRC, the world is your oyster. Thrown in a 5D?
5D is -9 on block. Might not be punished as hard as 2D would be.
5H is -4 on block, and is also special and jump-cancelable. So you can transition this mixup in to DandyStep mixups, IAD mixups or whatever.
For the most part, Slayer doesn't have a Dust setup. It's all conditioning and reads. 5H-5D is about the closest thing Slayer has to a Dust setup, and even so it still requires some conditioning.