I ran a test and this is the result:
Against Don Kixothe 'The Greek', with HP: 17,651 (single tank)
Ranged Combat (Nothing special happens)
Don Kixothe 'The Greek' missed Captured Minion's familiar
Don Kixothe 'The Greek' hit Captured Minion's familiar [1497]
Captured Minion's familiar looks weaker!
Captured Minion's familiar's Guardian Angel smote Don Kixothe 'The Greek' (256)
Melee Combat (Look at the decay damage)
HypnoToad's blow was dodged by Don Kixothe 'The Greek'
Captured Minion's familiar tapped Don Kixothe 'The Greek' [2074]
Don Kixothe 'The Greek''s Guardian Angel smote Captured Minion's familiar (374)
*** Captured Minion's Decay hit Don Kixothe 'The Greek' [10394] ***
Don Kixothe 'The Greek''s Guardian Angel smote Captured Minion (2882)
Don Kixothe 'The Greek' crushed Enslaved Minion [11477]
Don Kixothe 'The Greek' draws strength from his weapon! [101]
Enslaved Minion's Guardian Angel smote Don Kixothe 'The Greek' (112)
Don Kixothe 'The Greek' pulverized Enslaved Minion [10663]
R.I.P. Enslaved Minion, Captured Minion
The thing is, my Decay hits for more damage, than half of Don Kixothes original HP. Sure, he has a morg, but the HP returned isn't enough to give a decay damage of 10394, against what I recon is less than 17,651 HP.
Therefore, I think that the ToE adds HP, along with other things. Otherwise, decay vs. ToE is skewed.