I was just checking out Sona's stats on LOLking and only 29% of her players complete a lichbane during their games with her. Come on people, we need more Q->
pokes on 
. Having a Lichbane totally improves your KA/D and winrate and it lets you buy more stuff from more assists and KSes. You aren't playing the Maven until you are Lich Banein'.
I always get 
and 
first. Then 
then if I can I build or 
. It's not that I don't build it its just that I build it last and normally the game is over before then.
If I'm super ahead I get Lichbane. No point in letting them catch up. Usually have to be ahead in two lanes with no enemy fed for me to do this.
If I'm ahead/even, I get Banner. Teamfights can go either way even if one lane got a three course meal with a dessert. Rather have a super minion pushing bot/top while the rest of us duke it out in mid.
If I'm behind/even. Locket. More shields for everyone. Prevents some more damage and may turn a fight around.
I never felt like rushing Lichbane was a perfect go to thing for Sona, would rather get items like 
. Lichbane is really my "95% sure these guys are surrendering at 20, might as well make it 100%" item.
It's my rush item once I've finished a mana and support item. It's hilarious to see a 1/3rd health squishy/assassin think they have enough to take out the helpless Sona only to watch them pop with one empowered Q + AA and get the shutdown gold.
Buy it when you clearly have extra gold. Sometimes I face losing Sonas who decide to get lots of AP and they usually do outdmg/out kda their entire team - but they can't win. Sona carry is not a carry.
What about the runes and mastery?
Until you realize that shes a support and kda is irrelevant to supports, any support who brags about their kda, shouldn't be supporting imo. Granted, that doesn't mean I want to see 0/11/23.
I usually get an Ardent Censor or Zeke's first when I ever play Sona. Then Lich Bane. Then the other one that I didnt pick from Ardent and Zeke's. Then I get some Supportish items.
I agee though, Lich Bane is a good item on Sona to make targets easier to kill for allies.
I prefer Iceborn gauntlet over lich on sona since it gives sona everything she wants, mana, cdr, armor, ap and aoe slow. Lich is a good buy if you are ahead but if behind its not so useful.
Lich bane terrible idea.
