If you start as part of the Pugyel dynasty around Tibet in 1066, you will begin with a Famous level of splendor, actually almost halfway to Glorious. I played as Guge, but Maryul and Yarlung are other options. You’ll need to earn a little over 18,000 more splendor to become Legendary. I played a mostly peaceful game at speed 5, while breeding like a rabbit. Get 3 concubines and a wife with fertility traits and go crazy. I got up to 240 dynasty members at which point I was earning 14.5 splendor a month (you start in 1066 earning 3.6 splendor a month). After about 100 years I totally messed up and lost my kingdom and was reduced to a count, but there were so many Pugyels about it didn’t matter, I was still earning plenty of splendor. Achievement unlocked in 1227, along the way you should also unlock A Legacy to Last the Ages.