There is a thread of menace through the perception of druids for many, and rot sowers may be a big part of why. Treading the edge of heresy among druids, rot sowers pervert nature by introducing undeath to their magic. Some do this in the name of preserving nature, deeming the compromise of using necromancy to be a trivial matter in the face of the threat to the natural world posed by civilization. Others are less ideological and just take power where they can find it. Few are safe to be around or particularly friendly, but malice and violence are not strictly required to walk this path. Regardless of their intentions and personality, these druids are orders of magnitude more likely to seek undeath themselves (typically lichdom) than those of any other known druidic tradition.
At 2nd level when you take this archetype, you tap into some inky well of fell power. Spells of the Necrotic and Undead schools become druid spells for you. You also learn the chill touch cantrip, which does not count against your cantrips known.
At 6th level, you can take the energy of undeath into yourself. When you use your Wild Shape ability, you can choose to shift into an undead version of one of your normal forms. An undead form has immunity to poison and necrotic damage, darkvision with a range of 60 feet, and gains an expertise die on Intimidation checks, but only receives half the normal amount of healing from magical healing spells, healing potions, and similar effects. It is also vulnerable to radiant damage.
At 10th level, your connection to undeath allows you to forcibly tear the life from the earth. Once a month, you can perform a special ritual requiring you to suffer 2 levels of fatigue and 2 levels of strife and 8 hours of uninterrupted work to blight a wide swath of territory. Upon the successful completion of the ritual, all of the vegetation and creatures weighing less than 5 pounds within a 5-mile radius immediately die and begin decomposing into putrid, toxic ichor. Creatures weighing more than 5 pounds must make a Constitution save against your spell save DC or suffer a level of fatigue. Nothing will grow in this area again for a year unless it is ritually purified, which takes a 100gp of reagents and a month of 8-hour days of ritual work from trained spellcasters per affected square mile. At the Narrator’s discretion, other methods such as divine intervention, the hallow spell, or similar means may also be able to end the curse.
Also at 10th level, you gain the ability to conjure up a sinister fog that moves with you. Creating the fog takes an action and it lasts for one minute. The fog extends 20 feet from you and spreads around corners. You can see through the fog as if it was not there, but other creatures treat everything in the fog as lightly obscured. In addition, creatures in the fog other than you that are immune to necrotic damage are treated as resistant to it instead, creatures resistant to necrotic damage are treated as if they were not resistant, and creatures that are not resistant to necrotic damage are treated as vulnerable. Once you have used this feature a number of times equal to your Wisdom modifier (minimum once), you can’t do so again until you finish a long rest.
At 14th level, whenever you reduce a creature with blood to 0 hit points in an outdoor setting, you can summon a corrupted creature of grave earth and necrotic vines from a space adjacent to it. Treat this as a free casting of the conjure elemental spell cast with an effective 6th-level spell slot, except it can only conjure an earth elemental, the creature’s bonus damage is necrotic, and it has the undead type in addition to the elemental type. Once you have used this feature a number of times equal to your Wisdom modifier (minimum once), you can’t do so again until you finish a long rest.