Use once as a free action: flip a fair coin. Heads: your next draft offer rolls one tier higher and you gain a draft reroll. Tails: nothing but a sheepish flash on your king.
You draft Horseshoe Nail in either mode. Free action: you choose when to use it, and it resolves without costing your turn.
As a item card, it is a one-use consumable, drafted in either mode.
“For want of a nail, a whole tier was won.”