什么是四魔? What are the four "demonic" forces (maras)?

指恼害众生而夺其身命或慧命的四种魔类,即烦恼魔、蕴魔、死魔、天子魔四种。
From this reference:
https://baike.baidu.com/item/四魔

Or
The four maras (Skt. catvāri māra; Tib. བདུད་བཞི་, dü shyi; Wyl. bdud bzhi) are the four types of obstructive, ‘demonic’ forces (sometimes also translated as ‘demons’) which create obstacles to practitioners on the spiritual path. It is important to understand that they have no inherent existence and are only created by the mind.

  • the mara of the aggregates (Skt. skhandamāra; Tib. ཕུང་པོའི་བདུད་, Wyl. phung po’i bdud), which symbolizes our clinging to forms, perceptions, and mental states as ‘real’;
  • the mara of the destructive emotions (Skt. kleśamāra; Tib. ཉོན་མོངས་ཀྱི་བདུད་, Wyl. nyon mongs kyi bdud), which symbolizes our addiction to habitual patterns of negative emotion;
  • the mara of the Lord of Death (Skt. mṛtyumāra; Tib. འཆི་བདག་གི་བདུད་, Wyl. 'chi bdag gi bdud), which symbolizes both death itself, which cuts short our precious human birth, and also our fear of change, impermanence, and death; and
  • the mara of the sons of the gods (Skt. devaputramāra; Tib. ལྷའི་བུའི་བདུད་, Wyl. lha’i bu’i bdud), which symbolizes our craving for pleasure, convenience, and ‘peace’.

from this reference:
http://www.rigpawiki.org/index.php?title=Four_maras