Wan Shi Tong, Librarian can be a patient value engine or a large end-step threat, depending on how much mana you invest. His entry trigger places X +1/+1 counters on him and draws half X cards, rounded down. Flash lets you wait until an opponent's turn to decide whether that investment is better than keeping mana available for interaction.
There is a useful break point every two points of X. Increasing X from three to four adds another counter and another drawn card; increasing it from four to five adds the counter but no additional entry draw. When mana is tight, those even values help compare the immediate return with the resources you would retain.
Opposing library searches provide another source of growth and cards. Flashing Wan Shi Tong in response to a fetch land activation or tutor can let him see the search when it resolves. Each qualifying search adds a counter and draws a card, so the creature can continue growing without another large mana payment.
Flying and vigilance give that growth a practical use. He can pressure a life total while remaining available to block, which suits blue value decks that want their card engine to contribute to combat. Counter support such as Hardened Scales in a blue-green deck improves the size, though it does not increase the number of cards specified by the entry trigger.
The search payoff depends on opponents' actions. At a search-heavy table it can be a major engine; elsewhere, judge the card mainly by its flash body and entry draw. Keep enough mana for the X value that meets your immediate needs.
