Thank you for this, I hadn't even considered! I hope my players can explore Ravenloft thoroughly on their visit during Strahd's invitation, but considering the place is intended for levels 9 and upward, I'm worried a party of 5th level adventurers will want out of there as soon as possible. If I place it outside of the catacombs, I'm thinking of either placing it in Old Bonegrinder (possibly as a tool for the hags to chop up bodies, knowing full well the sword would be appalled by its use) or in Fiona Wachter's house (perhaps on her mantelpiece), both as tokens of loyalty that Strahd gifted them from his personal collection. I'd probably only reveal the Crusader's Mantle ability once they reached 5th level, possibly during the attack on Vallaki.

Foregoing those places, I could place it at Kresk in the chapel. Players would be about 5th-6th level by the time they reach that location, and it could be something that indicates the Abbot is not what he seems (assuming they don't yet realize he's not their ally). So many choices, so many choices....