What happens if I succumb to demonic temptation?

At one point in Brogue, I decided to run in a northwesterly direction (Shift+Num7). To my surprise, instead of running in that direction, the following message was delivered:

A dark presence surrounds you, whispering promises of stolen power. Succumb to demonic temptation? (y/n)

I've said No, to which,

The evil dissipates, hissing, from the air around you.

This was exceptionally unexpected, but it's also made me vastly curious. While I turned down the offer, I can bring it up again at any time by pressing the same key combination, to get the same promises and the same choice. This key choice isn't listed in the in-game help as far as I saw, so it's never quite explained what it does.

What happens if I do what appears to be selling my soul to demons? What is the benefits, the stolen power I receive? And what, naturally, is the catch, the drawback of doing so?

Solution 1:

This Shift-Num7 combination only works when Num Lock is off - so what you're really pressing is Shift-7 — the ampersand symbol on some keyboard layouts.

Anyway, the current version of the game explains the prompt by saying:

Succumb to demonic temptation (i.e. enable easy mode)?

Answering Yes gives:

An ancient and terrible evil burrows into your willing flesh! --MORE--
Wracked by spasms, your body contorts into an ALL-POWERFUL AMPERSAND!!!
You have a feeling that you will take 20% as much damage from now on.
But great power comes at a great price -- specifically, a 90% income tax rate.

It should be said that easy mode does not reduce the gold you find in the dungeon, but your final score.