If your Basil Knight guy doesn't have a psy rating, a Force Weapon should actually be quite inferior to plain old technology.

Remember it's apparently a Nemesis ''power'' lance. For when you want to break canon as well as balance.

For your assault marine attacking hordes are you remembering that his special ability for horde kills only applies to a charge attack when in squad mode not every attack he makes. Remember all his melee attack successes are then halved to work out how much damage he does to a horde. so his first round attack as he charges is only gonna do success/2 +1d10 horde damage (plus whatever felling does) and every other round he'll get maybe four attacks if he has twin weapon wield(melee) and lightning attack, but all these as i've said don't have the benefit of his Assault Marine special ability. At his level he really can't be a horde buster,

I've been playing an assault marine for two years (finally at rank 5), my maximum damage is around (depending on size of horde) five horde damage per hit, thats 9 successes per melee attack, luckily if i roll as good i'll do 15 horde damage per round (lightning attack) I suppose i could take a penalty for an off hand attack and do a bit more.

When I finally get Whirlwind Attack and get to double my successes per attack then I can call my self a horde buster

Felling doesn't directly affect Hordes, it would just affect their damage threshold in order to do Magnitude damage. Felling also has no effect on enemies that lack Unnatural Toughness.

Devastating, OTOH, directly increases hits to Hordes, as do power weapons.

How about have a great warrior from either the Deathwatch or the marines original Chapter arrive and humbly request the use of the relic/uber weapon for some great quest he is going on. Give the player a big hint that he should grant such a request (or just have the Watch Commander order him to if PC doesn't take the hint).

Tell the PC that in a century or so the weapon will be returned to him. Then give the player a couple of points of renown because the weapon he wielded is now being used on a quest of great importance (and to sweeten the pill that your taking away the weapon).

PC gets to be invovled in a bit of RPing and is remembered in game as one of the wielders of some awesome relic weapon.

Problem solved.

Plenty of way to hurt them...

I had a recuring Necron Lord They would bump into on occation... Fatepoint were burned the first time they meet him... Poor assault Marine who decided to hold him on his own to let the team flee, and then used his jump pack to jump away... Didn't know that the Triach had some thing similar...when they finally did manage to take him down with heavy fire, he just phased out.. Good times..

Or other Astartes... Don't just take the one in the book... make them from scratch and make them unque.. A devestator with a terminator and a jump pack... they won't catch him in melee... Make unique Marines they are quite memorable too.. Also while its tempting to use traitors (also so they won't grab their gear) it can be just as interesting to fight Marines that are not memebers of the imperium, but still fight to protect it... but won't budge on their own moral stance..

Or shoot them down, where their gear is damaged. Or a psyker get their weapons to jam... lots of oppertunity.