Rank 10 Hades with 70% win rate in support role.
I don't see Hirez switching him into a Mage, they are too busy making new gods to use time doing that.
How to fix him in my opinion:
Death from Below is fine as is, compared to its most similar counterpart ability (Bacchus's Belly Flop), it does a little less damage, but slow is stronger and is easier to hit and has a shorter cooldown, but no knock up of course.
Shroud of Darkness should apply Fear without needing to Blight. Instead, if Blight is already applied, it should reduce protections by 5/10/15/20/25 (something along those lines).
Devour Souls should scale more appropriately for a mage/bruiser role if desired. Simplest way is to increase detonate damage to gods like you suggested. Possibly a more conservative 65% would work.
Pillar of Agony needs a change the most and many ideas have been suggested. My thoughts are that one of the following should be added to the ability (not all): Cripple, Healing reduction, or Non-channeled placement of the "Pillar" (which is currently himself).
-Cripple forces a beads purchase much like Ares' Ult, because currently Hades' Ult is very weak to Leaping gods (which there are more and more of).
-Healing reduction adds huge utility and protects Hades during the duration of the ultimate especially in the late game where gods with lifesteal would simply hit your face to survive through it.
-Making it a non-channeled ability that is placed like a Zeus Ult for instance is my least favorite of the options but solves some of its problems. Protections would be removed since he's not the center of attention from it, but it could prove much more useful and be less of an "All-in" play, gives much more options to work with.
Overall I think he needs just a few little things tweaked to be more viable. One last thing that could make sense is replacing some of his insane mana scaling per level with more health scaling per level. He currently gains +80 hp and +53 mana per level, if 5 points were exchanged --> +85 hp and +48 mana --> he would gain 100 health at max level at the expense of 100 mana.