Mara.

བདུད་
Tibetan: dud
English: Mara or demon

The story of the Buddha's quest for complete awakening and fulfillment reaches its climax under the Bodhi tree. It is here that Siddhartha Gautama formed a firm resolve not to get up from his seat until he was enlightened.

When Siddhartha took his seat and entered into meditative absorption he entered into battle with Mara, who was trying to prevent and derail his quest.  Mythologically, Mara is a powerful demonic god, but metaphorically anything that exerts a negative influence and creates obstacles and problems on our spiritual path is considered a demon, or Mara. 

As a practitioner, Mara symbolizes our own self-cherishing and our preoccupation with the eight worldly concerns.  These are what we enter into battle with on our own path.  As these powerful armies rise up against us, we must sit firmly and not waver in our practice.  We must remain unhooked by these clever and mischievous demons.

We get hooked by hoping for pleasure and fearing pain.
Our minds ensnared by hoping for fame and fearing disgrace.
We become trapped hoping for praise and fearing criticism and blame.
We are totally caught up in hoping for gain and fearing loss.

All of these are the self-cherishing habit of ego, caught up in hope and fear, attachment and aversion.  These are what keep us stuck in the cycle of samsara. 

Waking up from ignorance and delusion isn't easy.  The path isn't easy.  At some point you need to turn and face your fear.  You need to learn how to work with these demons, see how they hook us.  We need to understand how they how they shade our perception and influence our decisions and actions.

These demons are not real.  They are intense, but they are not real.  Knowing that we can start to work with them.