Welcome to Librarium Online!
If I understand correctly, the enemy knows your character has the fellblade as soon as you roll a die for it at the end of t1... so you lose the element of surprise. Or am I missing something?
From the Nightmares of Mortals, For the Greater Good of the Devourer of Worlds!
...I think what your missing is the benifit of the weapon.
S10, ward saves must be rerolled, and causes D6 wounds!!! Lets face it, theres a very good chance your going to kill what you want when you get in combat (barring ASF or you dieing first), be it character or monster.
Ok so your opponent sets up opposit with his nasty uint with uber character facing your uber unit. End of first turn you explain why you rolled and now he decides to change his game plan so you dont get in combat with his unit. Now you have the oppertunity to run him around the battlefield on constant watch for your guy with that sword. Use it to your advantage. You can also skitter leap your guy into a possition behind his monsters/lone characters etc if you need to make him have to deal with your guy. If you take the sword in the first place its on a throw away character so expect him to die and you wont be disapointed.
On the other hand, just dont take it. thats ###pts better spent elsewhere like some warlock engineers with some nasty magic items.