Official Canon Edit
There are no official accounts of Ollanius Pius in imperial records, only many versions of legends told by the Guard. The true story, however, is slowly being told in the Horus Heresy series of novels.
Pius' true name is Ollanius Persson, though he is sometimes called "Pious Persson" as he's one of the few remaining Catholics in the galaxy. He is also a perpetual, who has been alive since ancient times and fought in Verdun in WWI. Other than being extremely old, he is an otherwise normal human being, possessing no superhuman strength like Vulkan, or psychic powers like the Emperor. By the time of the Horus Heresy, he had retired in Calth, but was present for the Word Bearers' betrayal of the Ultramarines, and re-enlisted. As the war progressed, Persson started having premonitions of his encounter with Horus.
Persson's story hasn't been told in full yet, but according to the legend, he was present during the Seige of Terra and somehow found his way into Horus' lair, in spite of its extremely hostile environment. During Horus' duel with the Emperor, Ollanius intervened and confronted Horus. Horus killed him using his Chaos enhanced psychic powers, though this gave the Emperor the chance he needed to kill Horus.
TTS Canon Edit
The Emperor clarified that while there were many conflicting stories of what exactly happened during his duel with Horus, the true story was that Ollanius was the one who sacrificed himself for the Emperor, and not wanting to be upstaged, an Astartes Terminator and a Custodes also threw themselves at Horus. The Emperor was proud of Ollanius Pius, but was unhappy with the Terminator and Custodes who threw their lives away and wouldn't let an ordinary human have his moment of glory.