Everything you need to know about Akeba's Signets and joining the Shadows
I've made a couple posts this past week about an issue that people have been experiencing with joining the Shadows. In a nutshell, if you are in an Adventurer Clan (the standard clan you can join when you start the game) when you begin the questline, the game prompts you to donate your two Akeba's Signets to your clan and makes it so that you seemingly cannot complete the "Gathering Shadows" portion of the questline. I've decided to lay out an explanation, guide, and fix for stuck players here, so that it's all in one place.
In reality, what is happening is that you cannot join the Shadows if you are in an Adventurer Clan because Shadow members can only be part of a Dark Clan. The reason the game prompts you to donate your Akeba's Signets to your clan is to help your entire clan convert to a Dark Clan. Once this happens, the "Gathering Shadows" line of the quest will proceed.
This being said, the game makes no effort to explain this, and if you're part of an Adventurer Clan with few members, inactive members, or members that don't pay attention to the Shadow Lottery, you're basically SOL...
If you're tired of waiting for your clan to collect enough signets to convert to a Dark Clan, you can join the Shadows on your own, but you'll need to leave your clan. Once you've done that, you can either begin the questline if you haven't already (it's pretty straight forward to complete if your not in a clan when you start it), or you'll need to click/tap the magnifying glass next to the "Gathering Shadows" quest in your log. When you do that, the "JOIN" box will be grayed out, but you can click/tap on it anyway. This will open a prompt that, when you choose "OK", will restart the questline** so you can join the Shadows on your own instead of waiting for your clan. You will not be able to re-join the clan you left until they convert to a Dark Clan. (**Results may vary on whether or not the game makes you wait for the server reset to be able to restart the questline.)
Once completed, you can join a Dark Clan and carry on your merry way toward whatever the hell becoming an Immortal entails. You can also continue to collect Akeba's Signets to give to players that aren't already part of the Shadows, though I'm unaware of any other use for them after you join.
Turns out that it's not a bug, but a really, really poorly explained mechanic...Fuente
FYI on Akeba Signet issues players are experiencing with Clans.