You don’t/can’t kill the Void Tendrils. But they calm down and are no longer hostile to the Knight when you acquire the Void Heart, I believe.

If you're wondering how to get the journal entry, these enemies are unkillable like Garpedes. You get a journal entry by inspecting things like an enemy's dead corpse, also like the Garpedes. For the void tendrils, to the right of the lighthouse is a secret room in the ceiling that you can only access with the shade cloak. There's a strange structure that you can inspect and get the journal entry from. Just look up the exact location because I've wasted enough of your time anyway.

Edit: nevermind i found it.