Rage. I like your Berserker Frenzy but after nerfing Battle Fury, I think multiple attacks rampage (no limit) is far too open to abuse. Any new attack still qualifies for Rage bonus (like with Battle Fury). As well as Bear Strength, Vampirism etc. The more elegantly simpler text that is less open to abuse would be "Gain Rage +1, Bloodthirsty +0 and Counterstrike. Treat friendly non-mage creatures as an enemy." Rage + Counterstrike is a huge combo, especially with Growth around, so leveraging the fantasy trope of Barabarian Berserker attacking its own allies makes sense. Perhaps make it cheaper 2+3 due to this drawback.
Okay. Given that Battle fury was errata'd, maybe that sort of effect should be avoided without some sort of counterbalancing penalty (I was thinking maybe taking additional non-enraging damage for each additional strike, but actually there may be a better way). Your proposed fix sounds like a different sort of spell altogether (something that might be used as a quasi-curse); maybe call it "bloodrage" or "confusion". Here is a counter-proposal for a change: limit the number of additional strikes to 1 but allow the player to boost the strength of the second strike. Also, new attacks do not qualify for the rage bonus, because rage gives melee +X, which only applies to the first attack made each round.
I like your Tree Sanctuary idea. Flying is probably a better idea than invisible; it is certainly easier to price.
Sometimes, like with the Simulacrum or Spatial Disjunction, the flavour text seems the germ for the idea. If this is the case, then here is a flavour text famous saying to inspire you: "You are what you eat".
I don't know if I would say that my ideas come from flavor text; generally they come more from figuring out how to implement interesting combinations of mechanics (e.g. spellbinding creatures = Magic bottle) or figuring out how to implement a fun effect from a different game (e.g. Borgnjor's Revivification = Reanimate Flesh). But here is an idea inspired by that flavor text:
Regarding Murkh's Belt: Currently, enchantments with the Upkeep +X trait are not paid for until they are revealed anyway, so I think it is a moot point. While an enchantment is unrevealed, it has no characteristics, and once it is revealed, you know what it is anyway.
And I agree that mana denial is not really warlord-esque, but he needs something he can use against enchantment heavy builds, given his lack of arcane skills.
Some more responses:
Dreamcatcher. Nice title and idea but I don't think it will see competitive play, even with Lotus. Problem is Sleep is psychic so situational, Idol of Pestilence hurts Sleep etc. Situational augmenting benefits are always suspect as they are "win more". If you have Sleep on so many enemies, you don't need the extra Channelling. It may be useful in some niche match-ups where both players use Sleep effects. Also, what is the Dreamcatcher? Why is it flammable? Keeping to the card title, the Forcemaster really needs extra creature actions to time her attacks so I think this card will be very useful if it said "Zone Exclusive. Unique. You control all opponent's creatures with Sleep markers instead of that opponent". I appreciate Nightmare below is "in the same set" but +X Channelling is a bit vanilla in comparison.
The ability to control creatures with a sleep marker is another think I have considered, but I think that may be the basis for yet another spell. As you imply, it is hard to keep creatures asleep. So what if this card also made it harder to awaken creatures, such as placing a damage threshold or requiring a d12 roll? You'd still have to make them fall asleep, but they would be less likely to get up.
Phase Barrier. I'm having trouble visualising what this Burnproof Acid Immune Force barrier is. Preventing damage out-of-turn during Damage and Effects step breaks the structure of the game (not even enchantments can do this). I also think you could create something more unique than a simple physical Force Wall by making it an Illusion. "Phantasmal Veil: Illusion Wall. Mind 1. Cost 4. No Access. No LOS. Extendable. Incorporeal. 1 Life. Once per round, before a friendly Action phase, Phantasmal Veil may lose its No Access and No LOS traits until the end of that round. Flip its Ready marker to Used to indicate this use." This retains your one-way accessibility concept whilst retaining the game adherence (so far) that walls apply to all players (here you only control when you open the wall to everyone).
Thematically, this is a wall of mental force (like forcefield). When attacked, the caster must concentrate to prevent the wall from disintegrating. The caster can phase out the wall to allow a friendly creature to fire through it. Nice idea with the illusion wall, although I'm not sure why it would block access. It does give me an idea for an enchantment that lets you create a secret passage through a wall, though. I will think about possible modifications to this ability to put it more in line with how other timing mechanics work while preserving the flavor.
Nightmare. Another really nice idea but I think this is too powerful as is. Firstly it needs Nonliving and all Incorporeal creatures so far have Upkeep +1. I also don't think it needs its "does not wake target" text. Attack on Sleeper is 3 dice critical with Sleep 7+ so has 50% chance of waking target (far more exciting). I'd add text "Mages suffer Daze instead of Sleep". Fast Flying makes it too much a creature assassin so I'd lose both, instead add text "when activated, pay 1 to teleport to any zone with a Sleep marker" as it interacts with any mage's Sleep effects. I like hybrid schools. I often feel mage books build themselves because spells are mainly in 1 school, like playing mono-colour in Magic, except you have to splash for essentials. However hybrid makes a card more expensive usually (Lord of Fire fits Warlock training perfectly). I always felt Bridge Trolls should have been just Nature 1 War 1 to be more played. So after nerfing the card above, I'd probably reduce it to Mind 1 Dark 1 (so costs Mind or Dark mages 3 points, rest 4 points).
Good point about the chance of target falling back to sleep; that simplifies things. Upkeep is probably a good idea too. Fast is unnecessary; the teleport to zone with sleep marker is a better implementation. Flying was intended to aid with survival, but I suppose incorporeal is already pretty good for that.
Haunted. I like all the concepts behind it, the creature paying for that murder by being harassed by spirits with haste. Obviously it needs Spirit support (like Shadow, Nightmare etc). It can be used with friendly creatures to create a walking spawnpoint like the Tome (except a cantrip crucially) and Warlock can collect both it (cantrip) and another curse via curseweaving. No quibbles here, just needs Spirit support, perhaps for any future Witch.
What opinion do you have about the cost of the card? It is a powerful spawnpoint, but it is also hard to reveal. Wasn't sure how to price it. Should it be cheaper?
Magic Bottle. Nice idea (as always). However, creating this quick cast summon mechanic may "let the genie out of the bottle" so to speak. I am not comfortable with this as it could make future creatures with coming into play effects too powerful if summoned via a quick action. However, you are spending a quick action preparing it (or Battle Forge) so it is a combo prep for action burst mechanic, like Enchantment Transfusion. So maybe it would do the game good to have more combo enablers. Out of all your ideas, this has the greatest possibility for abuse. I'm comfortable with potions not using up an item slot (the "only one" rule prevents abuse). I'm not so comfortable that dissolving it after it has 3 mana can trigger its summon. I would nerf the text to "You may bind any non-Epic creature from a trained School to the Magic Bottle. When Magic Bottle has at least 3 mana on it, you may destroy it as a quick action to summon the bound creature in your zone." This means Dissolve effects (Orchid) can still destroy the Bottle without letting out the creature otherwise there is an unstoppable inevitability of it just like with Enchantment Transfusion and current "move to counter targeting" FAQ rule. I limited it to trained School creatures so that Air Wizard can have classic Whirling Spirit, Beastmaster can bind a Grizzly into it etc. This is the one card here that could be a great combo enabler but needs thorough testing and by limiting the quick action to in-school creatures, we are retaining the mage's theme and limiting possible future abuse.
You're probably right. I'll make sure the mage has to break the bottle. In school creatures only is an interesting idea, though I'm not sure how to justify it thematically. One idea I was playing around with was letting the mage throw the bottle to summon the creature in an adjacent zone. Thoughts?