The Crypt of the Brothers is a building of the Lotus Clan.
The Three Brothers of the Lotus religion may have died long ago, but they have not abandoned their people. Reports vary wildly on the location of their desecrated Crypt; perhaps the Lotus summon it at need, or perhaps the Brothers move it themselves. When it appears, one of the dread three; Lythis, Sekh or Tausil, may be summoned, at a price in Yin energy.
The chosen brother rises from his uneasy rest to walk the earth in spectral form, draining stamina from his enemies to empower the faithful with grim sorceries. The Brothers have a special relationship to the Warlock and Master Warlock units, who have achieved a death-in-life akin to their own. To these mages, each brother can impart a different gift.
It is not difficult to dispatch these phantoms with sword or bow, but this merely sends them back to the grave. Dauntless, they will rise again and again, ever seeking vengeance on enemies of the Clan.
In-game, the Crypt of the Brothers features like a very early Keep for the Lotus, in that it allows them to summon Zen Masters much earlier than is normally possible. However, only the Three Brothers can be summoned from this crypt, and the summoning of each brother still requires the player to spend a point of Yin energy.
The building is vitally important for the acquisition of Lotus Battle Gear; with the exception of the Second Tier units' ability of cannibalizing peasants to gain a piece of Battle Gear, the Lotus Clan units can gain Battle Gear in no other way apart from the Three Brothers expending stamina to bless the unit with a piece of Battle Gear.