So here are some relics I have thought of, but I need some people to gauge the balance.
Polyphemus (Astartes Lascannon)
Heavy | 400m | S/-/- | 5d10+12E | Pen10 | Clip 6| Reload 2Full | Proven (3), Accurate, Laser Sight, Telescopic Sight, Master Crafted| 80kg | Req: N/A| Renown: Hero|
This lascannon has confirmed to be the end of dozens of Chaos Space Marines. In one instance, its most famous wielder, 1st Company Captain Marius of the Silver Talons, took down 22 Chaos marines by himself with the weapon, almost every shot fired at the head, until he ran out of charge packs. A century later, Captain Marius was initiated into the Deathwatch as a Watch Commander, bringing the Polyphemus along with him into battle. It has been centuries since Marius' death, but the Polyphemus saw to the death of many heretics as it is passed down to Silver Talons and the Deathwatch.
(I know you can't have 2 sights on one weapon, but screw that, it's a relic)
Solaris (Bionic Eye)- (Inspired by Bale eye)
Pistol | 10m | S/-/- | 2d10+10E | Pen13 | Clip 3 Reload Full | Laser Sight, Master Crafted, Special | 8kg | Req: N/A | Renown: Hero
This system is automated by a machine spirit at a near-heretical level. Gain rival (Inquisitors) and 10 Corruption Points upon installation. However, as a free action, you can fire what is the equivalent of a master-crafted Mars pattern Inferno Pistol once per turn. After firing for 3 times, you cannot fire for one turn to allow the machine spirit to reload the weapon. You can also manually reload it as a full action before firing for 3 times, but you still cannot fire on the turn you reload. The complex system makes it heavier and quite bulky mounted on a helmet, and can only be used with an astartes power helmet on.
(I am unsure if the bionic eye is broken, so if the general consensus is that it is, then I would lower clip size to 2 or 1)
The solaris is a technological marvel, the intricate craftsmanship of the weapon itself and the machine spirit near-impossible to replicate. The mechanisms are entirely automated by the machine spirit, thus gaining the accusation of the inquisition that it is heresy to employ such a weapon. The Adeptus Mechanicus, however, defends it ferverently. What both parties agree on, however, is its destructive power of a melta charge capable of piercing heavy armor. This relic remains a contreversial rift between the Inquisition and the followers of the Omnissiah.
Frostfang (Frost Blade)
Melee | 2d10+11E/R | Pen9 | Unwieldy, Power Field, Tearing, Razor Sharp, Devastating (1) | 30kg | Renown: Hero
The Frostfang is a gigantic astartes-sized two handed frost blade, with the power to rend the greatest armor apart and cut a ship in two. This legendary relic has been lost to the Space Wolves since the primarch Leman Russ has embarked on the journey he never returned from, as the Frostfang belonged to one of Russ' honor guards. The blade is so large that it is impossible to parry with, but its destructive power more than makes up for it. This weapon can only be wielded two handed, or one handed in terminator armor.