According to this http://wh40k.lexicanum.com/wiki/Executioner it can be anything you want? "Another power weapon belonging to the Eldar, this weapon bearing the name Executioner takes many forms - that of a heavy spear, a rune-encrusted sword or a great black scythe. Whatever its form, it is one of the most deadly hand to hand power weapons."
I would take the. "What ever it's form' as reffering to the spear, sword or scythe? As I also can't find any pictures of a poleaxe variant either? I've poked two people that know Eldar a fair bit more than I do and ones said no axe, and didn't know they could be anything other than the glaive. (Which is also apparent in the 40k wiki, which states it can -only- be a glaive. ) Though they did find die cast banshee models with axes- though none of them were Exarchs, which means they weren't Executioners. And leads me to think it's of questionable canonicity these days.
It's simple. Back in ye old days you only had "power weapons". They all did the same. Just ignore armour. In later editions of 40K they intoduced stats for melee weapons. So now having the correct weapon for the job became important. P sword: Strength (s) as user AP 3 P mace: +2 S AP 4, concusive P axe: +1 S AP 2, Unwieldy (you hit last) So when this change happend you could have any power weapon you desired as long as your model had that version of the power weapon. Very old second edition Banshee units had axes. So Banshee units where nasty because they would always hit first no mater what. But having an axe made you hit last. This made a big dilema on the Table top scene as having the best AP on a power weapon with no drawback made many players not happy. So after the 5th Edition of Eldar and some FAQ's Banshees had their unit entry changed from power weapon to power sword. Making banshees only have 1 weapon to "choose" from. Dire avenger Exarch on the other hand can take a power weapon. Giving you the chance to model any power weapon variant you want on him.
Yeah, I was talking specifically in regards to Imperial Power Fists. To be fair, whilst 'one per planet' rare sounds quite rare, you have to also consider how many planets there are in the Imperium. It's not so vanishingly rare that you wouldn't expect to see one on the battlefield. I'd say you might expect to see one for every 50-100 Astartes. If there are a thousand Astartes per Chapter then something like 10-20 Power Fists per Chapter. Some will have more, some will have fewer.
This version of Executioner sounds more like the generic piece of wargear from 2nd ed. Even the imp guard and orks could use it.
