Ebonwood Sister - KF Witch

It could, sure. Health gain is boring though, and death effects are already well and covered by FW.

The thing is, i don't really have any ideas, but maybe one will form. OP's ideas are pretty good in general, and i really like his themes.

A Coven is just a group gathering together for a purpose. A Coven of witches evokes imagery of a gathering to cast a spell collectively. Witches, especially those that draw their powers from the natural world, are thought of as being weak alone. The power of their Coven, so to say. Instead of the internal, innate power of a Wizard. The power of their spiritual connections to each other, and whatever external source they draw from. I live near Salem, and i have some Wiccan friends and family, so this idea feels pretty accurate.

That's what i think of anyways, and since we already have wizards as a class specializing in raw magical power (Magic, Fire, Electric, Frost, and raw damage), i feel that Witches should deal mainly with other elements (probably not Psychic but Sonic, Poison, Acid, and a focus in additional effects like de-buffs instead of high damage).

The idea of the K'tharis Coven sacrificing their lives or a portion of their health, to the forest for their power, isn't a bad one. It can be made more interesting though. The Witches of the K'thir Forest could have their abilities be health fueled, or they could gain significant de-buffs (like Short Lived) in exchange for power, and have effects trigger on death (like the werewolf idea).

The Coven ability of the K'thir witches could simply mitigate and spread the de-buffs/health loss across all members of the Coven, or those within a certain AOE. For example, say there was a health loss of 20 to activate one of their abilities. It could be evenly spread across all members of the Coven. A health loss exceeding the maximum health of any one member of the K'thiras Coven could be activated once enough members of the Coven are present (would be interesting combined with death benefits, like in the case of the werewolf). A de-buff of 10 turns could be spread across 3 members of the Coven for 3 turns (rounding down), and a de-buff like Short Lived could be extended by turns for each member.

There are lots of interesting things that can be done with this. A reduction in maximum health isn't something that's been introduced yet (at least i don't think), but would make sense with the theme i've described. It would also go well with the passive ability described by the OP.

Like i said, there are more interesting things that can be done than health gain.

 

I think 'boring' is a must, actually. PoxNora is a very complicated game as it is, and Elsari Coven is a very simple, somewhat boring mechanic. it is predictable; it is consistent; and it is almost impossible to misinterpret. That is what I was going for, really: something that would be easy to code and very easy to understand.

I am not against complex or "cool" abilities, but I think racials -- in general -- should be simple. Voils with Flying and Sonic are -- IMO -- the pinnacle of a clean racial.

Here is another idea, which is nothing like the previous one. It is a heal-on-death combined with a conditional mini-Bestow.

K'Thiran Coven (passive)
When this unit is destroyed, the nearest friendly Real champion within 8 spaces receives +6 Max HP, as well as one of this unit's Upgrades if it has a Coven ability.

The point is that when a Witch dies, their energy is passed to another being, along with some wisdom, if the recipient is a sister/brother in spirit. It reinforces the "communal" nature of witches.

 

Gross...

The game can be intricate without being needlessly complex. There's lot's of fat in Pox Nora, but interesting abilities shouldn't be shaved off. That said, if you look at my post, i actually built off of your suggested racial.

Though i actually imagined something like this.
"Whenever a champion with K'thiras Coven is deployed, friendly champions with K'thiras Coven gain +2 Max HP. This effect stacks and is increased by 1 for each friendly champion with K'thiras Coven in play."

or

"While this champion is in play, all friendly champions with K'thiras Coven gain a stack of X. This champion gains 3 + (X(X+1)/2) max HP." Here, having 5 units with K'thiras Coven out at once, would yield... 3 + (5(5+1)/2) = 3+(5+4+3+2+1)
The bonus increases for each member of the Coven in play, and decreases respectively if they leave play.

EDIT: Anyways, ideas thrown...