Metal flux synthesis in a low-melting eutectic mixture of lanthanum and nickel has produced a family of complex intermetallic carbide phases. La21Fe8M7C12 (M = Sn, Bi, Sb, Te, Ge) has a new cubic structure featuring tetrahedra of iron atoms capped with carbon on each edge. These tetrahedra are surrounded by a La/M framework and are therefore isolated from each other. The antiferromagnetic coupling of the iron atoms is frustrated by their ideal tetrahedral arrangement; this is evidenced by magnetic susceptibility measurements on the La21Fe8Sn7C12 analogue. Deviations from Curie-Weiss behavior begin at 100 K; variation in field-cooled vs zero-field-cooled behavior is seen at 5 K indicative of magnetic ordering. AC susceptibility data indicate that the temperature of this transition is frequency-dependent, behavior characteristic of spin glass systems.