You can give yourself Lilith/Mephisto by:
Downloading
[Only registered and activated users can see links. Click Here To Register...]
and
Downloading
[Only registered and activated users can see links. Click Here To Register...]
Then resource both above .rar into your clients /Resource/ folder, since these pet's have never actually existed past their base models you will need to create them by hand.
1. Create a Pet Card in dbo.ItemResource
1.1 Create new Strings for the Pet-Card in dbo.StringResource
2. Create a Pet Entry in dbo.SummonResource
2.1 Create new Strings for the Pet in dbo.StringResource
3. Create a Base-Stat entry in dbo.StatResource
4. Create several Skill entries in dbo.SkillTreeResource
5. Create several associate Skill entries in dbo.SkillStage
(all the above entries must be saved into the client via rdb as-well or they won't show in-game)
If you have done these things correctly you should now have a working Mephisto/Lilith.
If you need some help, just ask.