Lucian's E, at full rank, costs 0 mana

Probably some weird coding reasons. It might be super complicated to make it go from "10 Mana" to "No Cost."

probably for consistency so that it is easy to keep track of the cost as it decreases with rank. If it had no cost from rank one, it would be worded as "no cost" but since it decreases with rank, they prefer to keep the same word format for improved clarity.

Also, nice bait, I came in here expecting a Lucian QQ thread.

Because the other ranks have a cost, most likely. 0 cost is probably because the other ranks of the ability have a cost, while "no cost" is reserved for abilities that have no cost at any level.

OCD is OP.

kappa

is this a game breaking issue? it is the same thing and its not free initially so they cant code it like a no cost spell, making an exception for one ablity's tooltip is overkill

put this in report a bug

Probably easier this way for them and the smallest change can create some massive bugs.

yet graves can't have a cigar

But shouldn't it cost mana to maintain Riot's view that "Champions without mana shouldn't be able to do things"