Masters of Illusion - Book 1 & 2

They have been hiding in plain sight for thousands of years. You just have to see through them.through their avatars... Some are predatory, others visionary. And then there are those like Niven, who comes from a long line of guardians. But, no matter what the species, no matter how well they managed to survive on Earth, their ranks are dwingling. Many believe their collective racial diversity is on the brink of extinction.
And then there are few of those who believe the answer to all their problems lies in the stars-and that's where they should return, to their respective home worlds. And like all great plans, this one calls for a sacrifice that only one of the stranded explorers is willing to make.  Some would call it heroic. But others, like those whose world has hosted the cosmic visitors for millenia would have a different name for it-armageddon.