There's long been a tradition in Ethiopia that the Ark of the Covenant rests in a monastery in Aksum, in the northern part of the country. It's supposed to have been brought back from Jerusalem by the Queen of Sheba.
Certainly the Ethiopian Church is keeping something at a monastery in Aksum, and has been doing so for a good long while now -- at least since the 18th century, and probably quite a lot longer. The Ethiopians say it's the real, original Ark. But only one living person at a time, the official Guardian of the Ark, is allowed to see it. And it's quite difficult even to meet the Guardian -- he never leaves the monastery, and does not generally take visitors. (Apparently he has no name, just the title "Guardian of the Ark". Rumor has it that at least one Guardian, when appointed to the position, made a vigorous attempt to flee and had to be dragged back to Aksum.) Even Ethiopia's Stalinist dictatorship, back in the 1980s, left the Ark's alleged resting place alone. So the truth of the matter remains a mystery.
Now, OTL the British invaded Ethiopia in 1868 on a punitive expedition. They defeated the Emperor Theodore II and took and looted his fortress at Magdala. They brought back a number of interesting items, including a crown, a diamond-encrusted cross belonging to the recently deceased Patriarch of the Ethiopian Church, and a number of tabots -- tablets engraved in Hebrew with the Ten Commandments, said to be first-generation copies made directly from the contents of the Ark. Tabots are rare, and hugely important to religious Ethiopians; when one was returned from Britain a few years back, it was an occasion for national rejoicing.
So, the WI: the soldiers looting Magdala find... the Ark of the Covenant!
To keep this simple, let's say it's not the real Ark. In fact it's a very convincing 15th century fake, made around the time of the great Emperor Zara Yaqub for internal Ethiopian political purposes. But Victorian science would not be able to tell the difference -- the truth probably wouldn't come out until the advent of carbon dating, almost a century later.
So the victorious British army marches back to the Red Sea, carrying what pretty much everyone believes is the real, no-kidding Ark of the Covenant.
Now what?
(I don't think Victorian Britain is going to go nuts and start carrying the Ark around on military campaigns or whatever. Most likely it will end up in the British Museum, solemnly examined by scholars, while skeptics mutter under their breath. But I do think there could be knock-ons for Victorian religiosity and Imperial British self-regard... not that it needed any help.)
Thoughts?
Doug M.