Mental shield



Mental Shield is the ability to block out all mental abilities that affect the mind. Bella Swan is the only known person with this exact ability.

"The shield blew out from me in a bubble of sheer energy, a mushroom cloud of liquid steel. It pulsed like a living thing - I could feel it, from the apex to the edges."

- Bella on her absolute control of her shield

Description
Bella considered this power a 'glitch' in her head whilst she was still human; Edward had teased her that he could only read AM minds, and that she was on FM. There is known to be another vampire with the same general kind of talent, though with different characteristics. The reason behind this "shield" is that Bella has a "private mind", a place where no one can harm her. It also made Bella very desirable to Aro, who likes to "collect" vampires with special abilities for the Volturi guard.

Once she is turned into a vampire though it is identified as an ability by Eleazar, who has the gift of identifying others' abilities, though he was blocked by her power. This basically lead him to identify her power anyways.

With the help of Kate and Zafrina, Bella learns to control and use her shield, eventually shielding more than one person at once. This came in useful in Breaking Dawn  when she domes her shield to protect her family and friends, meaning they could not be attacked by Jane or Alec, or any of the Volturi guard. Bella also learns to remove her shield to allow mental powers to work on her, such as Edward reading her mind at the end of Breaking Dawn.

As a vampire, she can willingly shape the shield in any way she wishes. While her shield is out, an extra shield will be produced to protect her. Also, it works as a one-way glass; it can shield anyone from outside mental abilities while allowing those she shields to direct their powers outward. She also mentions that whoever she shields becomes a part of her, allowing her to sense whom she is protecting without having to look. When she shields Jacob Black and Sam Uley; she finds their packs shielded as well, because they are telepathically connected with each other in wolf forms.

Limitations
Though this shield is powerful, it has many limitations. For instance, it cannot shield against a physical attack, allowing for an intruder to attack directly.

Bella initially had a very hard time pulling up her shield, describing it as an elastic recoil. This obstacle mainly comes from her subconscious desire to protect her mind above all else. Once she learns to let go of that desire, she learns to use it fully and completely.

The only person to penetrate this shield is Renesmee, who can place images and thoughts in Bella's mind through a single touch. This is because she has the exact opposite abilities of her parents.

In Eclipse, it is explained why Alice and Jasper's abilities are thought to be able to work on Bella when all other vampires abilities so far have proved ineffective. Alice explained that her ability to see the future is only seeing outcomes based on decisions and not the thoughts behind those decisions, though it has been speculated that she actually sees the decisions. How Jasper's ability affects people is unknown, though it has been speculated that it operates by influencing brain chemistry.

She also can't block Marcus's ability to read relationships because he reads the emotional bonds between people; Alistair's magnetic pull to living things; Benjamin's control over the natural elements; James's tracking sense; and Siobhan's ability to affect the outcome of a situation.

Similar abilities
Renata of the Volturi has the power to produce a protective shield to deflect a physical attack. However, unlike Bella's, she can only implant a thought into her attacker's mind to make him go a different path.

Another similar power is Fred's ability to drive others away by implanting his attackers with a revolted thought. Since these powers work by affecting one's mind, Bella can easily block them.

Trivia
The published chapters of Midnight Sun reveal that Bella's father Charlie also has defenses against abilities that affect the mind, though noted not as powerful as Bella's since Edward could still hear murmurs from his mind. This implies that abilities are at least partly hereditary. Another example of this is Renesmee, who shows the opposite of her parents' gifts.