Knowledge gained from ancient tombs and dungeons will power Seferis within the White, Blue, and Black
For example, Skyward Spider's primary color pair would be white-blue
Basically, at any given rarity (and considering the era of the card in question), we must compare its current price to other cards at the same rarity (and ideally set) that have already peaked more than the others
Lets say the Rabblemaster player has some pressure on the battlefield already, potentially a Tarmogoyf, and each turn that passes allows them to get in for damage at a rate that is unacceptable to the Terminate player
The spellbook is a wonderful artifact store with cards like Gilded Lotus, Heraldic Banner, and Zephyr Boots
His core strength lies in repeatable card draw tied to opponent behavior