This book is 23 pages long, 1 page front cover, 1 page blank inside of front cover, 1 page title header, 1 page editorial, 1 page ToC and SRD, 1 page back cover and thus leaves 17 pages for the fellowship, so let's dive in.

What is the Fellowship, then? It's actually a fully written group of adventurers that makes for interesting adversaries for the PCs - either as foils or rivals. The first two pages give us and introduction for the group as well as a how-to-use and 5 sample encounters.

After that, we get to the group: Each of the members gets the full Raging Swan NPC write-up, complete with mannerisms, personality, hooks, distinctive characteristics and so on. The NPCs also come with their own b/w-portraits you can show off to your players -nice! This should, in my opinion, be the standard for NPCs and is just awesome.
Major Spoilers ahead, so players beware and skip to the end!

Still here? Good!

The fellowship is:

-Aurakraul (young adult green dragon, comes with a fully detailed hoard she wants to reclaim; CR 11): Nice twist on the greedy dragon trope.

-Dhoean Talthar (half-elven bard 4/ranger 4, comes with a hawk-animal companion; CR 7): Sociopathic mastermind of the Fellowship. I LOVE this character. Seriously, this guy is thriller-movie-villain material.

-Holg (half-orc ranger 8, comes with a wolf companion; CR 7): Holg has a nice twist to him that can be summed up in "Reincarnation gone wrong".

-Lafithel Traivanna (elven sorceror [gold dragon] 8; CR 7): Beautiful, pampered and petty, she is the spoiled, yet driven capricious member of the fellowship and comes with loads of roleplaying potential.

-Vola (half-orc druid 8, comes with a wolf companion; CR 7): A great twist on the "evil" druid, Vola actually desires to be the smart predator and takes an impassionate stand towards her deeds.

The fellowship, as presented, is rife with potential for roleplaying, tragedy and internal strife. They can act as foils or even allies for the PCs. Or, you could just throw them at the PCs as cannon fodder. you don't want to do that, though, as their write-ups are simply cool and very, very detailed, coming with a plethora of potential ideas to use them.

As a great form of bonus content, we get a two-page IC-journal entry of adventurers who slew the dragons hoard as well as an appropriately-scribbled-looking map to the unclaimed hoard - it's one-step-beyond content like that, that makes a pdf truly shine.
The pdf closes with the obligatory page to introduce DMs to reading the stat-blocks.

Conclusion:
Editing is good, I only noticed one minor typo. The pdf is extensively bookmarked and layout adheres to the beautiful and printer-friendly b/w-Raging Swan standards. The artworks for the NPCs rock and their detailed write-ups are well-worth the low price of the pdf. Due to the amount of praise I heaped on the characters, you can already gather that this is going to get a good rating, thus the following suggestions should be taken as being extremely nit-picky: I would have loved to see a a section on their group-tactics in a sample encounter. One of the encounters mentions how complex an encounter with the whole group would be and thus I would have loved some detailed round-by-round-sample tactic of them as a group e.g. ambushing the PCs. While not necessary, this (and minus the one typo) would have crowned the book with the full 5 stars - my final verdict will thus "only" be 4.5 stars.

Fellowship of the Blackened Oak is the next in Raging Swan Press’s line of GM’s Resource supplements for Pathfinder. Unlike some of the other supplements, this one is more than just a resource. At its roots is a group of seemingly different characters allied with an obsessed wyrm seeking her birthright. At one end is a group of NPCs or Pre-generated characters for use within virtually any adventure or campaign, while at the other end is the framework of an entire adventure or launching point for a campaign. This supplement is very similar to the TRIBES line of supplements in that it takes a 3-dimensional look at seemingly mundane NPCs and turns them into characters with depth like a standard PC.

The obsessed wyrm is almost comical in nature as to how she won’t give up her search to find and claim her birthright. This creature is presented in a similar manner to the other characters with a fully detailed background, applicable stats and everything one would need to bring the character to life. All together, this GM’s Resource is one very valuable supplement for an extremely reasonable price.

Fellowship of the Blackened Oak is a very valuable supplement. Not just for the content but for the sheer number of opportunities that are presented to a GM to create interesting encounters, great adventures and interesting campaigns. It really takes a 360-degree look at a group of NPCs that can be implemented in a number of ways. They could simply be encountered on their own, used as main antagonists, combated as a group or become the source of a delve into the birthright of their wyrm ally.

For GMs looking to grab some quick information to add, enhance or build upon their adventures or campaigns, Fellowship of the Blackened Oak is a fantastic source. Everything about this supplement feels complete; the content covers almost every known aspect of this group of allies.