The lanthanum crystal Cs2LiLaBr6 (CLLB) has excellent scintillation properties for gamma/neutron discrimination. The initial melt with hyper-peritectic ratios (55 < LiBr/mol.% < 61) was found beneficial for CLLB crystal growth. The hyper-peritectic melt would pass though the peritectic point. However, the detailed process was still unknown. Herein, the dendrite growth and peritectic reaction were discovered near the peritectic point in CLLB single crystal ingots for the first time. The mechanism of dendrite growth was analyzed. The Cs2LaBr5-LiBr phase diagram was modified according to these results. The understanding of phase transition and dendrite evolution can guide crystal growth of CLLB and other similar elpasolite crystals.