WARNING: The following contains spoilers for Locke & Key Season 2, now streaming on Netflix.
Locke & Key Season 1 established the Locke family as having a rich history and legacy in Matheson, Massachusetts. Season 2 further elaborates on that history by delving deeper into the family's roots going as far back as the American Revolution in 1775. The first episode of Season 2 opens up with a flashback depicting the Locke family as blacksmiths developing tools and weapons for the American soldiers during the revolution. Though not soldiers themselves, the Lockes of the 18th century were still loyal to the people of their colony and supported independence from Britain. This got the attention of Frederick Gideon, one of the higher-ranking soldiers in the British army.
One night that the Lockes were getting ready to supply weapons to the American soldiers, Gideon showed up at their doorstep with a few of his own soldiers to inquire about their activities. The Locke family patriarch denied any involvement in developing weapons for the American militia, but Gideon doesn't take him at his word and has his men raid the Lockes' home. They bring out the Locke women as well as boxes of ammunition.
Being the sadistic soldier that he is, Gideon attempts to terrorize Locke by having his soldiers burn the women in his family. The Locke women are luckily saved by the timely arrival of American soldiers and a shootout occurs. Locke attempts to take out Gideon, but he is no match for the latter's brute strength. It doesn't take long for Gideon to overpower Locke and kill him with his own knife.
After being forced to retreat, Gideon and his remaining soldiers make it down to the Drowning Caves, where Gideon unwittingly locates the portal to the demonic realm. At first, Gideon is frightened by the portal as it seems to fire iron bullets of its own. Once he sees what the bullets can do once a person is struck with them, he begins to see the bullets as a source of power and voluntarily allows himself to be shot. Now possessed with a demonic entity, Gideon begins to experience physical transformations.
At some point, Gideon is caught and executed by American soldiers, but not before documenting his last couple of days in his journal where he talks about the portal to the demonic dimension, the whispering iron and its power. He leaves both his journal and whispering iron bullet behind, which then gets passed down to future generations of his bloodline, eventually landing in the hands of his descendent, Joshua Bennett.
While Gideon doesn't make any more appearances for most of Season 2, he is summoned as an echo by Eden in the final scene of the last episode, hoping to exploit him as a puppet. Gideon, however, quickly proves to Eden he's no puppet and disposes of her just as quickly as she summoned him. With a new chance at life and a new purpose, Gideon sets out for the town of Matheson.
Nothing is known about the terror Gideon will unleash on Matheson citizens and especially the Locke family in Season 3. However, based on what is known about him in Season 2, three possible scenarios could play out. The first is the most obvious: he'll most likely seek revenge against Locke's descendants for his capture and execution during the American Revolution.
Another possible scenario is that he will attempt to re-subjugate the United States to British rule, especially as a soldier who is loyal to the English crown. However, upon realizing it's been centuries since America got its independence from Britain, he could also decide to take the country for himself. If that last one, in particular, plays out, this could make him very similar to Dodge, who also attempted a demonic uprising in the United States as part of his goal of ruling the world.
Locke & Key Season 2 is streaming now on Netflix.