It might be the one for defeating LR 5 times.. mine is bugged too.
I have the other 2 already, and my progress bar says 92/100.
However, mine does not say "?". The text for it clearly says "Unlock this by defeating Lord Recluse at the end of a Patron Arc."
So maybe Lanaru's is bugged also but says "?" instead. Have you (or your group) done the Shadow Shard TFs in SG mode at all?
If it's the third badge listed on the Defeats tab, it's for defeating Lanaruu the Mad five times. (The order of those three badges is flamethrowers, Lord Recluse, Lanaruu.) The description is "???" and the bar itself is bugged in that it doesn't progress evenly for each time you defeat him. For example, my SG has defeated him twice and the bar is at 64%. I'd guess that you still have one or two times to go to complete the badge.
Blondeshell (1381 badges) - My other badge hunters
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I was looking at the list of my Supergroup's badges and noticed under Defeats that there was a ? counter that was at 96/100. The only Supergroup Defeat badges listed on Paragon Wiki and BadgeHunter are the 4000 Flamethrowers, defeat Recluse 5 times, and Lanaru 5 times.
"You don't lose levels. You don't have equipment to wear out, repair, or lose, or that anyone can steal from you. About the only thing lighter than debt they could do is have an NPC walk by, point and laugh before you can go to the hospital or base." -Memphis_Bill
We will honor the past, and fight to the last, it will be a good way to die...