MTG Salvation

Your reasoning is around backwards, in regards to X's. Let me answer the questions first.

If you control 16 artifacts including Mycosynth Golem, then its affinity for artifacts ability might let you cast a Hangarback Walker for up to as much as X = 8 while you pay no mana. The reason for that is that X = 8, which is a choice you make, results in Walker's mana cost being 88=16. When you factor affinity in, and you "pay 16 less to cast that spell", you arrive at a total cost of .

Notice that the cost you pay, depends on the value of X and also on the affinity ability. The value of X depends on nothing but your announcement of a value for it. The cost, specifically the mana cost or base cost, derives from your choice of X.

Affinity applies a cost reduction, which reduces the total cost you pay. It doesn't apply to (only) an {X} in the cost.

Yes, it triggers. Upon turning the card face-up, it has a triggered ability that cares about this event, so it triggers. (603.10)

The ability on Horizon Stone is


This replacement applies when the turn-based processes of ending a step or phas cause you to empty your mana pool. (106.4) "Instead", the mana becomes colorless. This replacement can repeatedly preserve mana in your mana pool across unlimited turns. Specifically, as the ending phase ends, instead of emptying your mana pool, all that mana becomes colorless.

The Release Notes for OTJ (found at this link: https://magic.wizards.com/en/news/feature/outlaws-of-thunder-junction-release-notes) say that at least one additional cost must be chosen to cast the spell.

Spree spells are modal spells and the lines given are modes. You must choose at least one mode of a modal spell as part of casting it.

Gaining double strike does not lose first strike. For the purposes of the combat damage step, having double strike supersedes first strike in importance, but the first strike ability is still there.

If there is an effect that makes a creature lose first strike, and Kwende, Pride of Femeref is also present, then this presents an interaction of continuous effects where there is a "dependency". (613.8a) That creature loses first strike before Kwende's effect applies, and as such it is forestalled from applying. (613.8b) That creature will not have double strike.

Nothing described here is a replacement effect, and first strike and double strike do not create replacements themselves either. A replacement watches for an event of some kind to almost occur, and changes that event to be something else before it happens. (614.1)

Each player other than you creates a token that's a copy of Phage. Its ability triggers for all those players, since they didn't cast that token from their hand. In reverse turn order starting ending with the player after you, they lose the game. (101.4)

When you sacrifice a creature that is both black and green, it triggers (not: activates) both abilities. When you would next receive priority, you put those abilities onto the Stack in the order of your choice.

Counters do not change base power or toughness, so Vrestin actually does have power greater than its base power.

208.4b Some effects check a creature’s base power and/or toughness. These effects see that creature’s characteristics after applying any characteristic-defining abilities and abilities that set power and/or toughness, ignoring any effects and counters that modify power and/or toughness without setting them. See rule 613, “Interaction of Continuous Effects.”

When your Insects and/or Vrestin deal combat damage to your opponent, that all counts as one event for Kutzil because they get folded into the quantifier "one or more". They deal the damage at the same time as each other.

Castle Garenbrig says you can only spend the mana to cast creature spells, or activate abilities of creatures. Where it says spells it means spells, and where it fails to provide another suitable word like "card, spell, or source", it means an object of the type creature on the battlefield. (CR109.2)

Reinforce on Wren's Run Hydra is an ability of a creature source if it's activated, but it is not an "ability of a creature" except when that Wren's Run Hydra is on the battlefield.

Cards in your hand are not creatures. Inquisition of Kozilek talks about "nonland cards", which allows it to refer to cards that do not have the type land. Any reasoning about creature cards in your hand is unrelated.

If the opponent has Troyan, Gutsy Explorer, then Drana and Linvala has this ability:

Activating this ability ability requires tapping Drana and Linvala to pay the symbol. You will also be restricted by summoning sickness if applicable.

Drana and Linvala also gains this ability:

Activating this ability requires tapping Drana and Linvala and also paying one mana of any type. You will be restricted by summoning sickness if applicable.

Quote from zeebo »

Let’s say that counterpoint targets a creature i’m trying to cast and i give it hexproof, does the second half of counterpoint still resolve?

Hexproof on a card in a graveyard does not apply. It doesn't do anything. The counterpoint can resolve in that construction.

No. The creatures return at the same time, which means the event of the Metamorph entering the battlefield is to take place before the Overseer is on the battlefield. Because the Metamorph copying is a replacement effect, the application of that effect, and choice of permanent to copy, has to exist before the entering event occurs. There is no Overseer to copy.

If a creature named "Gollum, Obsessed Stalker" has dealt combat damage to a player this game, then that is a fact that remains true for this game at all future points.
Note that if Gollum's ability is not on any permanent at the beginning of the end step, nothing triggers and nothing happens. Gollum still has to exist to make anything of the 'mark' as you put it.

Protection from a color, let's use the example black, means that there are a few rules for the permanent with the protection ability.

1) All damage from black sources that would be dealt to it is prevented
2) Nothing black can be attached to that object (Auras, Equipment, and Fortifications)
3) Black creatures can't be assigned to block that permanent
4) Black spells and abilities of black sources cannot legally target that permanent

There is an acronym to remember this list, DEBT, for Damage, Enchanting, Blocking, and Targeting.

Creatures that were disguised, cloaked, manifested, or morphed can be turned face up, using the appropriate action and paying the correct cost, any time you have priority. That's any time in the game at all that you could cast even an instant spell and it definitely includes during combat after blockers are declared.