A nanofilament made by spinning ether (the fifth imaginary element). For generations the Eltnam family has employed this filament to hack people’s brains, and make the knowledge of the hacked target their own. This modus operando has earned them the name “spirit hackers.” After connecting to a target just once, the filament can be extended up to 5,000 meters. Sion uses etherlite to incapacitate, attack, and steal knowledge among other things.

MB Official Strategy Guide: Melty Blood Glossary

The nanofilament wire wielded by Sion.
Although it’s used like a whip in the game, it’s actually designed to attach to the opponent’s skin, integrate with their central nervous system, and read their thoughts and memories. It is because of this technique that the name Eltnam has become synonymous with spiritual hacking.
Though the thought-reading function seems vile enough, its ability to “merge with and copy memories and experiences” is actually even more dangerous. This inhumane and demonic “acquisition of another’s life,” which even the Atlas Institute shuns, is a technique unique to the Eltnam family.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary