As a guy who no longer dabbles in 40k, someone has to explain this Alpharius thing to me. I see the name pop up continually in the meme world, but I'm missing the requisite context.
Alpharius is the Primarch of the Alpha Legion, who both in-universe and out-of-universe have gained a reputation for invoking his name Spartacus-style.
Other than what has been said by @ChapterAquila92 , Alpharius is known (both in lore and in tabletop) to be kinda elusive. He and his legion are masters of subterfuge and infiltration. You will never know for sure if you're facing Alpharius or just an Alpha Legion's officer that's impersonating him, while he's somewhere else, dressed as a regular marine. Hence my "Alpharius, is that you?"
"It's the wrong trousers, Horus, they've gone wrong! Stop them, Horus, Stop them!" *Cut to Tzeentch operating the trousers via a remote control... because of course Tzeentch would be the one who'd do it*