I don't think the main relay can make things fade out. Power for lights doesn't even go thru the main relay at all.
Sounds like your alternator crapped out, but that doesn't explain why the battery tested good! How was it tested? Open-circuit voltage might still be OK (like simply using a voltmeter). The real test is how does the voltage look while it's supplying a load of, say, 100 amps. If you don't have a load-tester, try measuring battery voltage with headlights & stuff turned on. All the stuff you can possibly turn on, the battery shouldn't drop below maybe 11.5v.
Check all the alternator connections for looseness or corrosion. You might be able to charge your battery with jumper cables, then drive home without headlights (daytime). If your alternator were working, the battery voltage would be about 14v or so while the engine's running.