Now let's go through the shadow functions.

DISCLAIMER
I don't believe that shadow functions are that important for our personalities. If you're here for self-typing then you probably should not rely that much on this post. Shadow functions can vary in everyone; even more than our primary functions. I, for example, didn't relate to them at all because some of the functions that are supposed to be in my shadow stack are more developed than those in my primary stack. (My Te is more developed than my Si)
In my opinion, you can type yourself without knowing about the shadow functions at all. But it's still good to know because it can be used as a reference while typing. It's also good to know about shadow functions because then you'll understand what other people are talking about.
This part won't be as detailed as the previous parts.

What are shadow functions?
According to Carl Jung, our shadow side is the repressed side of ourselves and the functions are undeveloped and unvalued.
Our dom and aux functions are the core of our ego (our identity) and also tert and inf by the time they develop, The shadow functions come in to protect the ego when it's not doing well on its own, for example in stress. They balance out our egos.
Let's take a look at ENTP's full stack:

Ne - Dominant/Hero
Ti - Auxiliary/Parent
Fe - Tertiary/Child
Si - Inferior

Ni - Opposing role

Te - Critical parent
Fi - Trickster
Se - Demon

Notice any patterns? The shadow stack is the same functions and order as our primary stack but is reversed on the introverted/extroverted axis. Now let's talk about what these positions and names mean.

Shadow function #1 (5th function) - The opposing role
This function acts in opposition to your dom function, it questions the way it sees the world and its goals. This is often the part of you that is stubborn, argumentative, and hard to get along with.
Our experience with this function is mostly negative, that's why we can see other users of this function as oppositional, passive-aggressive, or have hidden motives.
If used positively, this function can be used to back up the dom function, but it's quite rare.

Shadow function #2 (6th function) - The critical parent
This function shadows the aux/parent function. While the parent function supports and protects, this function humiliates and criticizes. It can feel other people's weak spots and go for them in a fight but it can also out our weak sides and criticize our ideas.
Sometimes when we see someone being childish we use this function.
When used positively this function can be used to understand others more deeply.

Shadow function #3 (7th function) - The Trickster
This function shadows the tertiary/child function. In contrast to the innocent and playful child function, this one can be malicious and sly. The Trickster wants to protect your inner child.

Because this function is so underdeveloped, using it can feel very uncomfortable and wrong; it can often lead to bad decisions and it's often untrusted.
When using this function we may feel that we're disrespecting rules or being manipulative; it feels wrong.

Shadow function #4 (8th function) - Demon
"Our inferior's inferior function."
This function feels the most unknown and unused to us, this function can be very destructive and leads to regrettable actions.
When we see someone using this function a lot we can project the worst qualities of this function upon them because our relationship with this function is so strained.
This function is deeply unconscious and can erupt from our minds unexpectedly.

Shadow stacks for each type:
ESTP: Si>Te>Fi>Ne (ISTJ)    ESFP: Si>Fe>Ti>Ne (ISFJ)
ISTP: Te>Si>Ne>Fi (ESTJ)   ISFP: Fe>Si>Ne>Ti (ESFJ)
ENTP: Ni>Te>Fi>Se (INTJ)   ENFP: Ni>Fe>Ti>Se (INFJ)
INTP: Te>Ni>Se>Fi (ENTJ)    INFP: Fe>Ni>Se>Ti (ENFJ)
ENTJ: Ti>Ne>Si>Fe (INTP)    ENFJ: Fi>Ne>Si>Te (INFP)
INTJ: Ne>Ti>Fe>Si (ENTP)   INFJ: Ne>Fi>Te>Si (ENFP)
ESTJ: Ti>Se>Ni>Fe (ISTP)    ESFJ: Fi>Se>Ni>Te (ISFP)
ISTJ: Se>Ti>Fe>Ni (ESTP)   ISFJ: Se>Fi>Te>Ni (ESFP)