The Moonshadow's Price
So at the beginning of the novelisation of Book 1 we learn that the reason why the continent was divided.
So, in episode 1 Aaravos recounts the dividing of the continent and as he does so we see him with a group of elves, two of which are female.
While either one of these elves is a good candidate for this merciful elf I'd like to propose that it is the Moonshadow elf who was the one who cried for mercy for the humans.
I believe that when she proposed this act of mercy, Luna Tenebris, a Moon dragon and therefore a creature with a deep belief and respect for balance decided to spare the humans. However, while the Moonshadow elves would have understood and accepted the queen's judgement the other elves and dragons did not. They couldn't understand this need for balance and quickly became enraged, defiant and divided. They accused her of favouritism, that she valued the voices and opinions of those connected to the Moon over everyone else's. So in order to keep the peace and ensure that her decision was honoured and followed a compromise was reached.
The humans could have the western half of the continent on the condition that when the elven border was drawn it must not include the Moon Nexus. This was the sacrifice that the Moonshadow elves and Luna Tenebris had to make to ensure that the humans lived. They and they alone would be cut off from their most sacred site and the magic that it contained.
So in order to ensure that the Nexus was never abused or found the Moon Druids destroyed the henge and appointed a mage to guard and protect their Nexus from prying human eyes.
(Note: Shirtless Runaan - refer to final paragraph for this to make sense. Sorry guys)
Luna Tenebris, heartbroken by this decision and with her reputation damaged beyond repair abdicated, allowing Sol Regem, to take the throne.
Meanwhile the Moon Druids, now separated from the Nexus forever needed to find another purpose. Then they came up with an idea, one which would give them a new purpose as well honouring the decision that Luna Tenebris made to spare the humans.
This was how the assassins were born. The Druids needed to find a new way of life, something that gave them balance and honour. So they became assassins who would honour the act of mercy shown by their kin by eliminating the worst of the humans, the ones who seek to create imbalance, the ones who choose to ignore and disrespect those who saved them through the use of dark magic.
She is the first to become an assassin
And her descendants follow in her footsteps, serving Xadia and it's people by killing the worst of the humans, thus ensuring that this elf and Luna Tenebris' mercy, her Judgement of the Half Moon is upheld.
1000 years later and this female assassin's descendants are still doing so. Runaan, the most Moonshadow of Moonshadow elves and a direct descendant of the first assassin ensures that balance is upheld, just as his ancestors have done for the last millennia.
However, Runaan makes sure never to forget his ancestor's sacrifice and to honour them and their once sacred duty he paints the Moonhenge symbol, once worn by his druid ancestors, directly over his chest, not only as a remembrance but as a constant reminder of the balance that must be upheld.
However, with the balance in Xadia now hopefully changing for the better, the time of the assassins is coming to an end. Runaan will have to find a new purpose, just as his Druid ancestors did so long ago.
What could that be? My guess is that Runaan, along with Ethari will become the new Nexus guardians who repair the Moonhenge and create a new group of Moon Druids, who use their once deadly abilities and magic to protect elves, dragons and humans from evil and dark forces as well as preserving their sacred site and educating those who wish to learn about Moon magic, Moonshadow elves and their culture.