When were bastards invented?

I do a lot of historical research for my books, even though they are Fantasy. While researching something totally different, I realized that bastards, or at least our concept of them, didn't always exist.

Bastards are fairly common in Fantasy literature. Both Amira and Thadred in Daindreth's Assassin are considered illegitimate children, though the circumstances of their births were vastly different.

Legitimacy or lack thereof is also a big topic in HBO's hit House of the Dragon series.

But there was a time in medieval Europe when the social status of a person's parents mattered way more than if said parents were married. No one seems to have cared about the marriage part, actually. The Ancient and Medieval Worlds had plenty of kings and princes who were the sons of mistresses/concubines.

Prior to being conquered and incorporated into England, Welsh law considered any child acknowledged by their father to be legitimate. Well into the 12th century, we see people whose parents were clearly not married being recognized as heirs under the law.

So how did we go from that to some countries having laws against "illegitimate" children receiving inheritances? Why were actual wars fought over who was legitimate and who wasn't?

I suspected it had something to do with the church, but oddly enough, the idea seems to have started with the secular courts using church doctrine in inheritance cases (though medieval separation of church and state was shaky at best).

I suspected it had something to do with the church, but was wrong. Oddly enough, the idea seems to have started with the secular courts using church doctrine in inheritance cases to define "legitimate" marriage. (Though medieval separation of church and state was shaky at best). 

In short, all the scandal, social stigma, and generational trauma surrounding children born out of wedlock is because a bunch of rich people decided to play dirty in court.

In the case of illegitimacy, the secular courts created cases around "illegal" marriage and the church band-wagoned onto it. They even created doctrines around bastardy.

Prior to the 12th century, no one really cared. William the Conqueror's parents weren't married, but when his parentage was attacked, it was because of his mother's social status, not because she was his father's mistress.

That means historically accurate Fantasy books inspired by the Viking Age, most of the Crusades, Ancient Rome, or anything else before the 1100's wouldn't have bastards. There might be people of unknown parentage and that would be scandalous. But the idea that someone couldn't inherit or was a subclass person because their parents weren't married was invented in the latter part of the medieval period.

Tell me about your favorite bastards in Fantasy! Have you read any Fantasy books were there were no "bastards"?