A molecule of pancuronium
Ahhh, this one.
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Pancuronium bromide is perhaps the most well known of all of the NMBs for its infamous and controversial role as the second ingredient of the lethal injection in the United States. It is officially a steroid structured nondepolarizing long duration neuromuscular blocking agent with the chemical formula C35H60N2O4.
It was first created in 1964 and it was approved by the FDA on October 24th, 1972.
People don't like pancuronium for the lethal injection because if the first drug doesn't knock them out like it should, they are fully awake to experience the painful respiratory arrest that pancuronium causes while being unable to alert anyone else to the pain. And then they get potassium chloride, which also apparently hurts very badly. I've never injected myself with KCl to find out, but I'd imagine the feeling of your heart stopping wouldn't be a pleasant one.
When used in surgery, it increases the heart rate, and it also can bind to pseudocholinesterase, which can increase the length of the depolarizing block caused by succinylcholine when the two are given together.
Pancuronium is featured in this awesome documentary (click here) about the lethal injection, and Carol Weihrer, who founded the Anesthesia Awareness Campaign, tells what it's like to be given it. Apparently she didn't like it very much. She also tells of pancuronium feeling like "ignited jet fuel" when injected, and this could definitely be true, as the steroid structured NMBs do indeed cause pain upon injection. Bisbenzyltetrahydroisoquinolines are apparently fine and do not cause pain, but I lack the experience to prove this.