SEN is a special mechanic in BLACKSOULS 2 which can be described as a meter of your sanity. If your SEN is 31 or higher, your entire perception of Wonderland will change and you’ll basically become delusional (or mad like the Cheshire Cat says).
When you start your game you will start at 100 SEN.
If your SEN is 0 or lower, you’ll be able to see the Wonderland for what it really is and most of the characters in the game will become noninteractive because the majority of them are mad.
You can lower your SEN levels by consuming Black Souls, killing certain NPCs, using other special items, and etc. While searching dead bodies and raping can increase it. There’s also a valuable item you can get by killing JubJub – Gear of Madness, which makes you able to configure it on your own whenever you want.