The Fall of Naxxramas (25 player)

If you want to do this, but do not have the achievement, or want to join on an alt of yours - and the group is requiring the achievement to join, simply copy paste this in a chat bar or put it in a macro. It will generate a link that will show that you have done it on 20/4/2009. You can just shift click the link to send it to someone else, it will not disconnect you.

/run DEFAULT_CHAT_FRAME:AddMessage("\124cffffff00\124Hachievement:577:"..strsub(UnitGUID("player"),3)..":1:4:20:9:4294967295:4294967295:4294967295:4294967295\124h[Heroic: The Fall of Naxxramas]\124h\124r")

Update - Correct link is now

/run DEFAULT_CHAT_FRAME:AddMessage("\124cffffff00\124Hachievement:577:"..strsub(UnitGUID("player"),3)..":1:4:20:9:4294967295:4294967295:4294967295:4294967295\124h[The Fall of Naxxramas (25 player)]\124h\124r")

The other one does not work anymore. This new link will once more show that you have done it on 20/4/2009.

Enjoy!

Gz to the one who downrated Shadowtion.. reposting the link in case the post gets deleted

/run DEFAULT_CHAT_FRAME:AddMessage("\124cffffff00\124Hachievement:577:"..strsub(UnitGUID("player"),3)..":1:4:20:9:4294967295:4294967295:4294967295:4294967295\124h[Heroic: The Fall of Naxxramas]\124h\124r")

This is the level 80 raid, not the level 60. As we can see, it's written (25 person) in the achievement title.