Winter, Misanthropic Guide Winter, Misanthropic Guide warps the game with a whopping two extra draws per turn
Not only is he expensive to play, but he's also expensive to buy
Our mana rocks like Astral Cornucopia, Everflowing Chalice, Empowered Autogenerator, Mindsplice Apparatus all generate more mana as we proliferate, our storage lands like Dreadship Reef also become huge rituals, Vat of Rebirth and Lux Cannon get way more effective, we can speedrun through sagas like Phyrexian Scriptures and so much more
The most famous example of this was the Mirage card Daring Apprentice that had errata after the prerelease but before the set's release (it needed to say the ability could be played as an interrupt it made sense back then)
You are able cast either one and unlock it, meaning its effect happens right away
Aaron and I loved this card