Humans and Sylvans, they look compatible…
In the case that such races mix, the Half-Sylvan is created.
Half-Sylvans cannot reproduce offspring, they are inherently sterile.
They look like humans and are practically indistinguishable from humans.
Half-Sylvans can be found out by testing their eyes in the darkness, the tapetum lucidum reflects much more than a human.
In some cultures, Half-Sylvans are put to death, This includes the more hard-lined Itarian cultures. Most cultures enslave Half-Sylvans.
The Former Kingdom of Reork was open to Sylvan/Human cross relations, however impractical it was to sustain lineage with such relations.
This led to a lot of Nobility whom chose such relations, to have alternative partners to produce offspring with, in order to pass on lineage and their holdings to.