Watchdog

Defensive artifact built as a colorless tax on attacking into it. The forced-block clause is the obvious cost (the body has to throw itself in front of something every combat it can), but the real engine is the static -1/-0 it stamps on every creature attacking you so long as it stays untapped. That untapped condition governs everything: the penalty only holds while the Watchdog is standing guard, so it evaporates the moment something taps it down or its controller wants to swing with it. The shrink applies to the whole attacking team at once and before damage, which is what makes it more than a lone chump-blocker. Against a board of 1/1 tokens it is brutal, neutralizing the entire swarm's power; against a single larger threat it merely shaves a point. The forced-block clause cuts the other way too, since the opponent dictates the matchup: a 1/2 throws itself in front of whatever is sent at it, and anything with base power three or more (reduced to two) simply kills it. So it is less a wall than a deterrent, asking the attacker to either commit a body big enough to push through or wait until the dog is tapped before alpha-striking. A narrow, honest piece of fortification design from an era when colorless creatures rarely carried static defensive auras, offering team-wide attrition to any color willing to run an artifact in the slot.