Note: I'm going off the 5E GK codex, which I hear has higher costs.

Juggernaut
This ancient suit of lost Grey Knight Terminator armour has become possessed by a modicum of Be'lakor's power, and now crusades against it's old brothers, with an ironic preference for human allies

Juggernaut can be taken as a HQ choice for: Space Marines (Codex, BA, DA, SW, excluding GK), Astra Militarum, CSM and CD. It may only be taken in the primary detachment, and cannot take the following as allies or supplements: GK, Inquisition, Militarum Tempestus, Adepta Sororitas, DA, Eldar, Officio Assassinorum, Orks, Necrons, Tau, Tyranids.

Ws 4
Bs 4
S 4
T 4
W 3
I 4
A 2
Ld 10
Sv 2+

Special Rules: Fearless, Preferred Enemy (IOM), Psyker Mastery 2 (Telepathy Only), Call to Darkness (All friendly units within 6" use Juggernaught's leadership for all purposes, excluding AM orders. These units also gain preferred enemy (IOM)).

Wargear: Terminator Armour, Daemon Hammer, Storm Bolter, Psybolt Ammo.

Cost:
GK terminator: 40 pts
Daemon Hammer: 0 pts
Psybolt Ammo: 20/5 = 4 pts
Mastery 2: (25*2)-"Brotherhood of Psykers" = 45 pts
Preferred Enemy boost and +1 Ld: 5 pts (taking a leaf out of the CSM codex)
Call to Darkness: 20 pts
+2 Wounds: 20 pts
Sub-total: 134 pts
Cool tax: 10%

Total: 150 pts.