When you finish the quests for your alliance, and the main quest, taking Cadwells Silver will move you to the 'territory' of another alliance.

This will allow you to get more sky shards helping for further skill points, complete the 'other' alliance quests to continue levelling and also gaining Champion Points and locating further lorebooks to help levelling Mage guild further.

A few things to be aware of: you of course don't run the main, mages or fighters guild quests in the new alliance again, you can't collect crafting writs in those zones, you are not in the same zone as players who rolled a character in that alliance only an instance of that zone for your alliance e.g. If you are DC you'll move to Auridon on Cadwells Silver but will share the zone with other DC players on Silver, not with any AD players who have just created a new character.

After Thieves Guild goes live you can can choose which alliance zones to move on to as part of Cadwell's but will only ever be in a zone with your own alliance players. I believe that the group finder will however start putting you in groups with players from other alliances though

and new headgear if you complete cadwell's gold

Fruitmass wrote: »

Johngo0036 wrote: »

and with the new dlc the soul shriven motif if you complete cadwell's silver.

Johngo0036 wrote: »

and new headgear if you complete cadwell's gold

Do you happen to know if those are going to be given retroactively for those who've already finished Cadwell's or will we have to go through Silver/Gold again to get it?

Supposedly retroactive.

OP, your questions have pretty much been answered, just going to add that currently the only way to group with players from another alliance is groupfinder. If you have friends on other alliances you can trick it into grouping you with them if you start the search at the same time. This will only work for group dungeons and it will automatically kick a person from group if they enter Cyrodiil. Aside from that, only interaction with players from other alliances you can have is that in aforementioned Cyrodiil and it's usually not a friendly one

fetito666 wrote: »

Thank you all!

I thought that some weeks ago I saw an upgrade in the Crown Store to be part if any Alliance, but I cannot find that item any longer.

What was it called again? Can I unlock it also by playing the other quests?

I think you mean the Explorer's pack or whatever it's called. I don't remember which section of the store it's in but it simply allows you to use any race in any alliance. So for instance I'm an EP Breton and you could easily be Dunmer AD or whatever with it.
It won't affect your actual alliance or interaction with players of other alliances, it'll simply let you choose any race you want when creating a new character(without it you're limited to the 3 races in your alliance, for EP it's Dunmer, Argonian and Nord).