What the raid boss eggs are, which of them are needed for clan skills, how to turn Destruction Tombstones into a Cradle of Creation at an Adventure Guildsman, and which eggs are rare on the market.
Clan eggs, or raid boss eggs, are the items needed for learning clan skills (Clan skills: how they are learnt and who they work for); they drop only from raid bosses, and there are no other ways of getting them (there is one exception, of which below), so we will farm raid bosses long and tediously. Only A-grade items are suitable for learning clan skills; the C and B-grade ones we either throw away or exchange at the Adventure Guildsmen in various towns for C and B gear and weapons.
The first item you will come across is the 
Destruction Tombstone. It drops from practically all raid bosses starting from level 40, so you will have very many tombstones. With the Destruction Tombstone another item can be got at any Adventure Guildsman.
We give the Adventure Guildsman 3 (three) Destruction Tombstones and get a chest, 
Adventurer's Box - Cradle of Creation, from which with some chance (by a double click) the raid boss egg 
Cradle of Creation can be got.
- Everything else drops from raid bosses:
Egg of Earth: A-Grade, 
False Nucleus of Life: A-Grade, 
Skull of the Dead: A-Grade, 
Dragon's Heart: A-Grade and 
Angel's Soul: A-Grade; on the page of each item there is the list of the raid bosses it drops from.
There are rather rare clan eggs and there are more widespread ones, so the prices on the market will be uneven: the cheapest will be the tombstones and the skulls, and everything else is dearer.