Splitters are the shades of mortals who dedicated their lives to forbidden practices with increased effort. The few souls devoted enough to their work were chosen by Lord Hades to serve in Elysium.
When attacking, Splitters fire off a moderately fast-moving large orb of purple energy that can be destroyed with any type of attack, or when it hits a wall.
When this orb is destroyed, it releases 8 smaller orbs which travel away from it in an 8-way spread. These can also be destroyed with any type of attack.
'...There is such knowledge that the gods sought to keep well afield of mortal minds; and there are mortals who pursued that knowledge, undeterred, all their short lives. Many now decorate the lowest pits of Tartarus. A scarce few, however, demonstrated such a zeal and talent that the Lord Hades soon put them to work, throughout his realm.'
Prior to the Big Bad Update, Splitters were a variant of Wretched Witch.
The Splitter is the only type of enemy similar to the Wretched Witch who's Elite variant does not have a unique attack pattern.
If a Champion Splitter is spawned in an Erebus room or in the Final Boss fight, it will have a unique spread fire pattern, where it shoots 4 large orbs away from itself in different directions. (These orbs will all split into 8 other projectiles when destroyed.)
If the large orb is Deflected, the smaller orbs created from it will lack the extra speed and visual effects of a deflected attack, but they will still harm enemies rather than Zagreus.