If we did want to add a negative affix to discourage players from using runeword, adding minus mana would probably work best as Iron Wolves don’t use mana. We can add Prayer Aura to the runeword to add a source of healing to the merc for added survivability while also adding -50 to Mana as shown below. Updated OP.
Piety
Shield
Vex Ohm Lo
Required Level: 59
Level 9-12 Prayer Aura When Equipped
+200% Enhanced Defense
+1-3 To Fire Mastery
+1-3 To Cold Mastery
+1-3 To Lightning Mastery
+35-45% Faster Cast Rate
+5 To Max Fire Resist (Vex)
+5 to Max Cold Resist (Ohm)
+5 to Max Lightning Resist (Lo)
-50 to Mana
Changed name to “Piety,” thematic for prayer aura. Name is from unfinished runewords list.