This Horde takes advantage of the fact that hordes can only be killed by emptying their library and removing all their creatures from the battlefield... and the titans shuffle back into the library as soon as they are killed or milled.  Thus, functionally and flavorfully, the Eldrazi Horde cannot be defeated until the three titans have been exiled, unless they are otherwise neutralized and a stalemate occurs (or if you have some other "you win the game" effect). 

Be wary of the Endless One -- as Hordes have unlimited mana, it enters the battlefield with a million +1/+1 counters!  Better have a steady supply of blockers!