No. The Warband Tavern is a fan-made, independently-run project and is not affiliated with or endorsed by Blizzard Entertainment.
With your Battle.net account, the same login you already use for World of Warcraft. We never see or store your Battle.net password — sign-in goes through Blizzard's own official login page.
Blizzard's public API doesn't expose everything a character has — Warband Bank gold, mailbox status, played time, several crafting currencies (like profession Concentration) and per-boss kill counts are only readable from inside the game client. The WarbandTavern addon reads those and a small relay agent uploads them; everything else (mounts, pets, toys, achievements, reputations, professions, gear, raid lockouts, and more) comes straight from Blizzard's API with no addon needed.
A plan for what to farm right now. It takes every raid and dungeon boss that can still drop a mount, pet or toy you don't own, checks which of your characters haven't already looted it this reset, and ranks the runs by how many new collectibles you can expect. Raids and Mythic dungeons reset weekly, Heroic dungeons daily, and Normal dungeons never lock you out. Lockouts come from each max-level character's raid and dungeon history.
From the community-maintained database of the Rarity addon, matched to the exact boss through the game's own loot tables. Once enough Tavern users have obtained something, the item page also shows our own observed rate ("Tavern data: 1 in X"), worked out from how many kills it actually took people.
From WoW's own kill statistics for each boss, read in-game by the WarbandTavern addon (version 1.36 or newer) and uploaded by the relay agent. Every kill on any of your characters counts, including ones from years before you signed up. Drops that have no in-game statistic, like container or fishing drops, aren't counted yet.
Only if you want gold/mail/playtime/currency data to stay current. It just needs to run while you're playing — it reads the addon's saved data and uploads it in the background.
Most data (characters, collections, professions, achievements, reputations, raid and dungeon lockouts) syncs automatically on a schedule, and you can trigger an on-demand sync from the Account page. Gold, mail, playtime, currencies and kill counts update whenever the relay agent uploads, which happens after WoW saves its data (when you log out or /reload).
No — it tracks your own account across all of your characters, not a guild's roster. Anyone with a Battle.net account can sign up and track their own account; there's no guild membership requirement.
Join the Discord (link in the footer) and post there — see the Issue Tracker page for details.