Seffrin, Charming Sellsword

When Seffrin, Charming Sellsword enters the battlefield, choose a creature type.
{x}, {t}: Search target player's library for a
creature card of the chosen type with a mana
value of X, put it onto the battlefield. The target player shuffles their library. That creature gains Cumulative upkeep {1} (At the beginning of your
upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)