Transmogrifying Licid
The Licid mechanic is one of the strangest type-line experiments Wizards ever shipped: a creature that can fluidly become an Aura and back again, blurring the line between body and enchantment in a way the game's templating had to bend to accommodate. The whole cycle exists to ask a single question: what if a creature could choose, mid-game, whether it wanted to be a permanent that attacks or a permanent that buffs something that does? The "you may pay to end this effect" clause is the structural hinge; it means the Aura is never permanently committed, so the same card can be a chump blocker one turn and a +1/+1 on a relevant creature the next, sliding off a body about to die and re-attaching elsewhere. This particular Licid grants its host the artifact type, which is the actual reason to run it: it turns any creature into an artifact for whatever cares about that (artifact removal you control, type-matters payoffs, cost reductions), while the detachable design keeps the granted type from being a liability you cannot undo. The mechanic never came back in a serious way, partly because the rules overhead of a creature-that-becomes-an-Aura-that-becomes-a-creature-again was a templating headache the modern rules team has been reluctant to revisit. What remains is a curiosity that captures a specific late-1990s design appetite: a willingness to build cards whose very card type was a moving target.
