For whatever it's worth, here's how I ran the monster maths from 2014 DMG with your half-orc rune knight:
TOTAL CR = (9 + 7) / 2 = 8
DEFENSIVE CR 9
effective hp = 112 + 14 treating Relentless Endurance as DMG treats Undead Fortitude (for expected CR 5 to 10) x 1.5 Hill Rune resistances to...