What account do you need?
The Steam store page lists an MTG Account as a third-party account requirement and says it supports linking to Steam. Plan to be online for your first launch so you can sign in if prompted.
Can you play offline?
Steam’s single-player listing leaves offline launch and first-time sign-in unspecified. If you need to play without internet, check whether the current build lets you start the game and enter a solo session after disconnecting.