What is a 'pernicious' wrong belief? Under six major wrong doings (Abhithanani) in Buddhism,
(i. matricide, ii. patricide, iii. the murder of arahants (the Consummate Ones), iv. the shedding of the Buddha's blood, v. causing schism in the Sangha, and vi. pernicious false beliefs (niyata micca ditthi)),


  vi. pernicious false beliefs (niyata micca ditthi)


is mentioned in http://www.accesstoinsight.org/tipitaka/kn/snp/snp.2.01.piya.html.

What is the definition of a pernicious false belief and
why is it considered such a severe offense to be in the same level as other acts?
First of all, the commentary to that sutta doesn't use the term 'niyatamicchādiṭṭhi' for the sixth abhithana, it uses the term 'aññasatthāruddesa', which means 'proclaiming another teacher', i.e. setting oneself up as a student of a teacher with views outside of the Buddha's teaching, something an enlightened being cannot do (abhabba kātuṃ).
As to what the term 'niyatamicchādiṭṭhi' (pernicious wrong belief) means, the MN tika says:

ahetukadiṭṭhiakiriyadiṭṭhinatthikadiṭṭhiyova niyatabhāvaṃ pattā niyatamicchādiṭṭhidhammā.
Views such as the view of causelessness, the view of non-action, or the view of nihilism, when fallen into a state of certainty are called states of pernicious wrong view.
(MN-t 22)

Basically, the meaning is that any wrong view becomes 'pernicious' when it is clung to as a certainty.