The Black Swords used by the Templars are supposedly Heresy relics. The Soul Drinkers, from the novels by Ben Counter, are searching after an artifact called the Soulspear supposedly owned by Dorn.
It works like this. Captain Bob of the, say, Ultramarines, survives the Heresy. He owns the Fantastical Rainbow Blade of Macragge, which he got from the previous Captain on his death. Gulliman splits his Ultramarines up into dozens of smaller chapters, and Captain Bob is made Chapter Master of a new chapter, the Supermarines. And he takes his Fantastical Rainbow Blade of Macragge with him. When Chapter Master Bob dies, it gets given to the next Chapter Master of the Supermarines, and ten thousand years later, boom. Insta-relic.
If you want a more recent Chapter to have the Fantastical Rainbow Blade, then say that one day the Supermarines are told to donate geneseed to the construction of the successor chapter, the...Awesomemarines. Chapter Master Joe of the Supermarines names Captain Jerry the new Chapter Master of the Awesomemarines, and as a present, good-luck charm, whatever, he gives Chapter Master Jerry the Fantastical Rainbow Blade of Macragge. And so on, and so on, for as long as you want.