This lets the Totem of Undying texture to Latias's Gens 3-5 sprite shown in your Pokemon party in the games. The texture pack also changes Ender Pearls to the Soul Dew item Latias and Latios use. There is also a Gen 7 version of the totem, but you need to rename the png to totem_of_undying for it to work and rename the other totem to something else.
You don't need Optifine xdddd.