MArines have to use Equipment available to themso I would use weaponse provided and add Force quaity to them

Theydon't just have to be Great

Here's what I made up for a Dark Angel Librarian in my game.

Force Glaive, 1d10+3 ® Damage, Pen 2, 2-handed, Balanced, Force Weapon

Basicly I made a 2 handed version of the Force Axe, kept the damage the same and gave it the Balanced property to keep it within balance with other weapons.

If I were to make a 2 handed Force Great Weapon, I would make it so that it would do 2d10 ® damage, have a Penetration value of 4 and the Unwieldy and Force Weapon qualities.

Interestingly enough, in Rogue Trader there's a Great Weapon with 2d10 +0 pen 0 Primitive Unbalanced, with same damage as the sword. So it looks like the RT Great Weapon got 2 less penetration over the DH one, but is Unbalanced instead of Unwieldy.

Either way, took that over to Deathwatch, removed Primitive, gave it +2 dmg, 2 pen and Special as the Force Sword has and voila, we got:

Force Great Weapon 2d10+2 ®, Pen 2, 2-handed, Unbalanced, Special

However, I'm interested in how important it actually is to be able to parry as a librarian, considering you only got one reaction per round (right?). Any thoughts about that?