Mobility spell King Rust possesses, one of the crawler-derived abilities in his stolen spellbook.
Donut's Size-Up readout on King Rust ends with an indignant flourish: he has Protective Shell at level 15, plus Tripper and Puddle Jumper. "What a copydog." Tipid attributes the whole grab-bag to a necklace of Indelible Woe, which swaps the wearer's spells for those of crawler targets—and takes one of their five highest if the target dies.
Stolen crawler mobility spell in the orc king's arsenal.
Also appears in books you've already read — spoiler-safe to their endings: