Scion of Stygia

Flash on a body this fragile tells you the creature was never the point: the enters-the-battlefield tap is. Three mana buys an instant-speed tempo intervention, a Frost Lynx with a d20 stapled to it, held up as a phantom blocker, flashed in to tap down a would-be blocker before your own swing, or dropped end of turn to tap down an untapped blocker before the crackback. Rolling the effect gives it two floors and one ceiling: a low result still taps the target for a turn, a high result denies the untap step and buys a full one, so the worst case is never a blank. That variance is the leash. A guaranteed lockdown at this speed and cost would be a genuine tempo staple; folding the good outcome into a probability, with the low band still doing real work, keeps the ceiling from becoming the default. The trigger fires once as the creature arrives and never again on its own, which is the difference between this and a standing lock like Icy Manipulator: you get a single well-timed intervention, not a recurring one. It belongs to the family of cheap blue creatures that trade a modest frame for disruptive summoning flexibility, the tap doing the heavy lifting while the 2/1 exists mostly to carry it and, once the real work is done, to trade or chip in a point or two.