A new smaller font and less whitespace on each page means Mythras Core is 304 pages vs RQ6's 450-ish. The artwork generally is smaller than in the RQ6 book which probably helps get the page count down as well. I also think a smidgen of the art from RQ6 is left out ( but there are a few new pieces too ).
1. Introduction by Steve Perrin and the history of RQ excised. Replaced by shorter intro to Mythras.
2. Strike Rank is renamed Initiative Bonus.
3. RQ6 Page 93 callout box - Weapon STR and DEX Requirements - dropped (not necessary as it was concerning RQ before version 6).
4. Combat chapter -
- The Move Combat Action is rewritten.
- The Regain Footing Combat Action is changed to as per Mythras Imperative version.
- 10 new combat special effects added (7 of these are the ones from the existing Firearms supplement).
5. Magic chapter has all the stuff about runes on pages 165-168 of RQ6 excised.
6. Animism chapter -
- The callout box The Source of Spirits on Page 202 of RQ6 dealing with spirits and runes is excised.
- The pages that were in the RQ6 Creatures chapter detailing spirits are moved into this chapter instead.
- What were Fetch Abilities in RQ6 have been split out of the Fetch Spirit section and listed under a new section called Spirit Abilities (under the Designing Spirits section) - the intention being that it is a list you can pick from when designing your own spirits. Some abilities are renamed, some dropped and some added - there are now 31 of them.
- All spirit types are now associated with one or more of these Spirit Abilities. Each spirit type write-up now explains any special effects a spirit has as being one or more of these Spirit Abilities (this has minimal in-game effect compared to RQ6 - it is just a better system of codifying spirits).
- New Medicine Spirit added. Curse Spirit given full stats.
- The subsections; Spirit Binding, Using Spirits, and Summoning a Spirit have been re-written to varying degrees - spirits can now be bound to 3 things; fetishes, locations or creatures.
- The section Spirit Combat has been rewritten somewhat - spirits can attack in the real world (as well as the Spirit World) without first pulling the victim into the Spirit World.
- There are also now Special Effects for Spirit Combat.
- There is now a 3rd type of possession - Passive Possession
- RQ6 page 297 section Runic Affinities excised.
- All mentions of affiliated runes for sample organisations are dropped.
- The Giant entry has a new callout box: One Head or Two?.
- The RQ6 Appendix 2 and 3 tables for Chaos Features and Non-Human Hit Locations are moved into this chapter.
10. RQ6 page 456 - Recommended Reading and Viewing - dropped.
11. Some chapters have their contents re-ordered slightly to make them flow better.