In the voice of the Found And Explained guy

Imagine: You're a Takish fighter pilot, flying a routine nightly patrol mission, when briefly, out of the corner of your eye, you see something fly through the air, at speeds no aircraft could handle, let alone reach. You look down at your radar in panic, hoping that it has any trace of whatever that was, but nothing... And the next moment, your RWR flashes, and you desperately pull for an evasive maneuver, but it's no use...

This would be the average encounter against the legendary hypothetical Lasa fighter, Borealis (Or as the documents call it: M-1XX).

Information: The Borealis was a supposed 7th generation super fighter, which could reach speeds of up to (and some reports say past,) Mach 10. It would've achieved this by using a completely new type of engine, which used a fission-fusion cycle system (basically the engine would have tons of helium gas in it, which would first be broken down into hydrogen, and fuse back into helium, and somewhere in the process, some of the energy would be released, propelling the aircraft forward).
It would have also had extraOrdinary stealth capabilities, appearing practically invisible to both radar, and even the naked eye, by using some type of light warping invisibility cover. And it's offensive armament was no joke either, as it would've had a pulse laser mechanism for/instead of the main gun, and the L-R XX radar guided missiles for the ranged armament... And keep in mind... This was all in the 60's, and 70's

"Too good to be true" you might be thinking to yourself, and that's because it probably is, as this was only a rumored fighter, with no definitive proof of its existence, even with the "recorded" 200+ air to air kills it has...