(Disclaimer: I know this is more Christian ish folklore than Celtic but it's an Irish legend and may have even been originally a Celtic legend that got Christianized so I just classified it as Celtic)
Lore:
Stingy Jack. Jack the Smith. Jack o' the Lantern. By many names is he known, yet by none is welcomed or wanted. Stingy Jack lived a life of disrepute. He was a drunk. A deceiver. One of the dregs. Eventually, the Devil himself heard rumors of Jack's ill-repute and went to see if he lived up to the stories. The Devil was not disappointed, and he sought to take Jack's soul down to Hell. Through trickery and guile the man was able to best the Archfiend, and ten years later, he did so again. Upon his drink-wrought death, Jack was unsurprisingly barred from Heaven, but when he sought to make a home of the Pit, he found another set of closed gates. Who but Stingy Jack could be too horrible, too infamous for Hell to admit? What horrors and atrocities must he be guilty of that not even the Devil would house him? Condemned to wander the in-between with naught but a carved lantern to light his way, Jack's resentment for the housed and the welcomed only grew stronger and stronger, burning brighter than any earthly flame. Each year around Samhain the bonfires ward apart the merging boundaries, keeping Jack condemned and barring the gates to our world. But now that the war of the gods rages on, all gates flung open in battle, is there anywhere left that is not an in-between? Light your feeble bonfires, cower behind your toothy masks, and take up your flimsy, carved lights, for the night and winter are dark, and Jack o' the Lantern is condemned no more.
I like this general concept and am surprised there aren't more submissions for this character. This wiki page's story gives a very specific reason for Jack not being admitted into Hell, so I'm curious if the Irish legend version tells a different story.
I like the passive idea, but wonder if it would hurt him too much when not fighting near the middle of the map. One alternate concept that I think still would work would be to give him a stacking buff over time with max cap based on the amount of time he's in the jungle. I'd sort of consider the jungle as an "in-between" compared to the lanes.
For his crucifix, an alternate application could be that instead of throwing it, he could hold it up in front of him and it would do line damage with additional CC effect (and maybe a lighting effect)...could be his general clear ability.
On the Hell side of Devil's Coin, I think either an increasing % pen or flat pen might be better than just the base 5 flat pen, which is quite underwhelming.
Either way, some fun ideas here, well done.
The passive buff is definitely something I considered as using as the jungle instead of fountain distance, but then I worried that his passive would be virtually usesless on like joust/arena/assault maps. For the crucifx ability that's definitely a cool idea that I might consider using, and for devil's coin the only reason I left it flat and unchanging was because I was worried that along with the universal unbreakable invis it might have been too much in one ability, but if you think it would still be balanced with a scaling buff, then I'll def consider adding it after the contest is over :) THanks again!
So you could do something like 10% pen, which automatically scales due to increasing enemy prots as they level and build items, or you could do a stepped leveling...5/5/10/10/15 flat pen. Note that these numbers are just ideas of course.