Cane of Byrna
The Cane of Byrna is found in a cave on Dark World Death Mountain. This cave is accessed by jumping the ledge to the South after using the Dark World warp below Tower of Hera. To travel the cave and get to the chest containing the Cane, Link needs the Hammer, a lift upgrade, and to survive the spike floor, 14 full hearts or a way to mitigate damage taken (Mail upgrades or Cape) or some way to replenish lost hearts (potion or fairy).
The Cane of Byrna will make Link completely invulnerable to damage at the cost of magic. The magical ring that circles around Link after it has been cast can also damage some enemies or potentially lag the game in busier rooms.
When Link first casts Byrna, 16MP will immediately be used (full magic bar is 128MP). From there, 64 frames pass before Byrna sets into a pattern of using 4MP per 24 frames. This does make Byrna more cost efficient with magic than the very similar Magic Cape.
Byrna can be used with an Itemdash. It will be cast immediately, and Link will be invulnerable to damage during his dash.