Multiple stack abilities and casting spells in that time

Question

My question is as follows: If I have multiple "At the beginning of your upkeep..." abilities, and I want to cast an instant spell or activate an ability during that time, can I cast the spell between those spells, or only as respond to all of them?

FOR EXAMPLE: If i have Splinterfright and a Endless Ranks of the Dead under my control. Now I decided for no good reason, I want to cast Dark Ritual after Splinterfright ability resolved, but before Endless Ranks of the Dead did, can i do that?

Answer 1

Yes, it is possible to respond to one trigger after others have finished resolving.

After Splinterfright's ability resolves, the active player (you) gets priority to play spells/abilities. See rule 116.3b:

The active player receives priority after a spell or ability (other than a mana ability) resolves.

At this point, you have an opportunity to cast Dark Ritual. If no player has any further responses, then your Ritual will resolve, followed by Endless Ranks of the Dead's ability.

Answer 2

Yes. The Upkeep step goes like this:

503.1. First, any abilities that trigger at the beginning of the upkeep step and any abilities that triggered during the turn's untap step go on the stack. (See rule 603, "Handling Triggered Abilities.")

503.2. Second, the active player gets priority. Players may cast spells and activate abilities.

So at the very beginning of the step, all of the "At the beginning of your upkeep" abilities go on the stack at once. They're triggered abilities - this'll be important to know shortly. You'll have a chance to respond to them.

Since they all go on the stack at once, though, you don't get to do anything between any of them going on the stack, so as to insert effects between them. Instead, you'll have to wait until they begin resolving, and use your spell/ability with the right timing.

This means the order they go on the stack is important. That brings us to this rule on triggered abilities:

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 101.4.) Then the game once again checks for and resolves state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.

Emphasis added. This means if you have multiple "At the beginning of your upkeep" triggered abilities, you get to put them on the stack in whichever order you want.

Your opponent also does the same. The active player puts theirs on the stack first, followed by each other player in turn order, in case anything of theirs triggers. That's what the "APNAP order" bit means. This means the active player's stuff resolves last.

So as a play example...

If i have Splinterfright and a Endless Ranks of the Dead under my control. Now I decided for no good reason, I want to cast Dark Ritual after Splinterfright ability resolved, but before Endless Ranks of the Dead did, can i do that?

Here's what you do in this case. I'll assume nobody else does anything.

1. Place Endless Ranks of the Dead's triggered ability on the stack, then Splinterfright's triggered ability. The stack looks like this:
   • Endless Ranks of the Dead's triggered ability (bottom)
   • Splinterfright's triggered ability (top)
2. Wait for Splinterfright's ability to resolve. The stack is now this:
   • Endless Ranks of the Dead's triggered ability
3. Cast your instant or use your ability.
   • Endless Ranks of the Dead's triggered ability (bottom)
   • Your other thing (top)
4. Let your other thing resolve. Then let Endless Ranks of the Dead's ability resolve.