Are Darkin the weapon or are they trap in the weapon?

Kayn has a quote where he says "Did such foolishness lead to your imprisonment"

So ima guess that the Darkins were demon things that were sealed away into weapons and stuff.

Both are correct.

Darkins are trap in the weapon, and due that, they are the weapon.

Take Kayn's Darkin form for instance, they are the weapon, but they can become the creature by taking over a host

originally, they are trapped in the weapon, so technically they are not the weapon... however, they appear to be one with them...

My theory is that they were originally beings free from their weapons; they just wielded the weapon. However, they were sealed into their weapon and can't permanently undo the seal. This is why when they take over a host, if the host dies, they revert to the weapon.

{quoted}

I've always thought that they were trapped in the weapon.

Canon answer

Darkin are alien entities that descended upon Runeterra in hopes of conquering it. While they were nearly successful, they met heavy opposition which eventually eradicated their species. The remaining Darkin were individuals who became imprisoned and sealed away within weapons to contain them.

Because of this, they are now the weapon. Despite this, these entities still carry tremendous power and has the ability to corrupt anyone that comes in contact with them; and thus taking over their bodies.