The Banner of the Stag sounded like it would fit quite well as a fully fledged faction in 5e. So here it is. I make no excuses for blatantly plagiarising the Forgotten Realms equivalent – The Emerald Enclave.
The Banner of the Stag is a Nimmerian group of wilderness-warriors formed to protect the scattered communities of southern Nimmeria. As followers of Erastil they believe in preserving the natural order but not at the expense of stifling civilisation so long as that civilisation is simple and attempts to be in harmony with nature as much as possible. They are particularly dedicated to rooting out 
unnatural threats like the many mutants that stalk the land. The organization is decentralized, hardy, and reclusive. As an organisation dedicated to Erastil (some say founded by Erastil) the tenants of this wilderness religion are followed and most members are Lawful Good. Druids and rangers of good persuasion are commonly drawn to the Banner of the Stag.
Goals
• Restore and preserve the natural order.
• Destroy all that is unnatural.
• Keep the elemental forces of the world in check.
• Keep civilisation and the wilderness from destroying each other.
• Look to the preservation of isolated towns and villages from marauders.
Beliefs
• The natural order must be respected and preserved.
• Forces that upset the natural order must be destroyed.
• Civilisation and the wilderness must learn to coexist peacefully.
Member Traits
Members of the Banner of the Stag are spread far and wide, and usually operate in isolation. They learn to depend on themselves more than others. Survival in a harsh world also demands great fortitude and mastery of certain fighting and survival skills. Having said that members of the Banner are dedicated to helping others survive the perils of the wilderness so are social, though they may still spend months in the wilderness to defend their respective towns.
Ranks
- Spring Warden (rank1)
- Summer Strider (rank2)
- Autumn Reaver (rank3)
- Winter Stalker (rank4)
- Master of the Wild (rank 5)
Iadrin “Redfang” Ashworth is the main organising force in Nimmeria for the Banner of the Stag and Iadenveigh is their headquarters. Redfang would be the recruiting point during the Iron Gods Adventure Path for characters interested in joining.