The stake part from the original Combat Cross was known for staking vampires, which is how it got the nickname "Vampire Killer." When Gabriel shattered the weapon & left the pieces in the shadow dimension, the stake that he used to stab that bitch Carmilla was still intact within the castle.
I'm assuming Alucard will use it to forge his new sword, then that sword might get damaged somehow & then rebuilt to form a new Combat Cross, the true "Vampire Killer."
That said, I really don't like how they approached it story-wise. They should've never have made Gabriel shatter the Combat Cross in LoS1; instead, he leaves the shadow dimension with it but simply discards it, as he no longer has a use for it anymore. A brotherhood knight finds it, gives it to Trevor, etc. What I'm getting at is, "The holy weapon I once used to defeat the Lords of Shadow & Satan, is now used against me." For me, that gives it a much better mythos, rather than there being multiple Combat Crosses as seen in MoF. They should've kept the weapon special & unique, and why only a Belmont can wield it.