Essentially, the target must discard one card per black mana used, making it a super powerful option capable of winning games for players
Obviously this is pretty much the best case and a lot of that glory comes from the Bloodbraid Elf
Tymna is a top commander choice because she has the Partner ability, which - like Tymna herself - was introduced in the Commander 2016 set
OP4 included a brand-new package of Animal Kingdom Pirates cards in the blue colorway, making them easily slotted into your old Kaido deck, breathing new life into it
The Eldest Reborn falls into the latter category, combining multiple card advantage-generating effects into a near-guaranteed plus three for the player using it
The best way to handle this is to look at the Goldfish Metagame page and look at how much decks of your preferred format cost on average, and set your rental limit accordingly