So I remember reading somewhere that half-elves and half-orcs are sterile and cannot possibly have children.
Then there's this eldritch heritage thing.
I have a player who is playing a human with eldritch heritage (orc) and I am trying to understand the family tree.
If he has an ancestor who is a half-orc, and that half orc had the ability to have children, then it all makes sense, if not then I am all sorts of lost.
No, just looking at the description for the races. Just look at half elves here and the discussion of the 'skilled' alternate racial trait for half orcs discussing 'second and third generation half orc' in this link here. There is plenty of supporting evidence to that they are races that can support their own populations.
So, for your question- no, there is no problem. Of course, I do kind of like the idea that this is still kind of a problem within these communities. That sterility and genetic conditions are common place among them seems a bit natural given their status as hybrid races. But this is more of a setting and roleplaying concern.
Rules wise- you are fine. It vaguely seems like the general assumption is that you can mix with just about anything that doesn't outright forbid it. Heck, have you even seen the undead and verdant (plant) bloodlines? (I know, there are more ways for bloodlines to get tainted with eldrich forces...but part of me just likes to think that humanoids have 'reputations' among the various sentient forces of the universe)
You remember incorrectly. Half-orcs and half-elves aren't mules.
They can mate with other members of their race and procreate. You just so happen to be able to form a creature that qualifies as a member of the race by having parents of the respective races.
Half-elves
Elves have long drawn the covetous gazes of other races. Their generous life spans, magical affinity, and inherent grace each contribute to the admiration or bitter envy of their neighbors. Of all their traits, however, none so entrance their human associates as their beauty. Since the two races first came into contact with each other, the humans have held up elves as models of physical perfection, seeing in the fair folk idealized versions of themselves. For their part, many elves find humans attractive despite their comparatively barbaric ways, drawn to the passion and impetuosity with which members of the younger race play out their brief lives.
Sometimes this mutual infatuation leads to romantic relationships. Though usually short-lived, even by human standards, these trysts commonly lead to the birth of half-elves, a race descended of two cultures yet inheritor of neither. Half-elves can breed with one another, but even these “pureblood” half-elves tend to be viewed as bastards by humans and elves alike.
May have been another edition or a book somewhere, thanks for invoking the search-fu guys.
I believe it was in 2nd Edition (perhaps Dark Sun) that the offspring of Humans and Dwarves were almost always sterile. They were called "muls", a derogatory nickname akin to "mules".In Arcanis the offspring of dwarves and humans were gnomes. Since dwarves were a cursed race of celestial giants, the curse would be passed down as an extremely hideous deformity. As such in both dwarves and human societies gnomes were at the bottom of the social ladder.
That's an interesting factoid I did not know.