Prime Evil Soul Shards will be for helms, while Lesser Evil Soul Shards will be for weapons.
Hearkening back a bit to Diablo 2 and the socketable skulls, Season 25 will have Soul Shards you can socket into your gear that are literally pieces of the seven Evils, with three for the Prime Evils and four for the Lesser Evils. You want to use the powers of Hell itself against the Demons of Sanctuary, right? Good, because Season 25 of Diablo 3 is going to let you do just that, as we pay homage the 25th Anniversary of the Diablo franchise by stealing the power of the Evils to use against them, in the form of Soul Shards, a new type of Legendary gem only available during Season 25.