The Phantom overheard Scrooge mentioning that the new safety lock of the Money Bin responded to sounds. Specifically to "My Bonnie", an old Scottish ballad, when played on a fife. (Fifes are small, high-pitched flutes). He proceeded to steal Scrooge's fife with the intend of stealing xash from the Money Bin.
Attempts by Scrooge and his nephews to locate and capture their opponent backfired. Leaving them trapped in various locations of the Cathedral. The Phantom was very agile, had acrobatic skills and had the benefit of fighting in his home grounds. He was only apprehended when the Ducks used General Snozzie to track down Snozzie's scent.
They were surprised to find that the Phantom had mostly taken petty cash from the Money Bin. Not caring about the fortune there for the taking. His motivation was simple. He had spend decades of his life creating a model version of the Cathedral, entirely made of the coins thrown in the wishing wells by visitos. He wanted a chance to finish his masterpiece without having to wait another decade. Scrooge simply agreed to help him. On one condition. The Phantom had to reveal its face. Only for Scrooge to tend up staring at a mirror image of himself.