How to determine initiative for a summoned creature entering an ongoing combat?

Question

Lets consider that a creature is summoned during an ongoing fight:

1. Does it roll initiative normally?
2. Does it have to wait a whole round to act?
3. Can it act before other creatures (assuming it's got high initiative)? Even if it means acting before creatures already engaged in combat?

Answer 1

1. Yes
2. No
3. Depends: If the summoned creature rolls a higher initiative than the one who summoned it, then it must wait till next round before it acts. If it rolls lower than the summoned then it acts as soon as it's imitative comes up even if that means going before others already in the fight.

In general, a summoned creature, and any unknown creature at the beginning of the encounter, roll normally for initiative and wait till its turn comes up for it to act. In these cases, its actually advantages to roll lower than the initiative of the person doing the summoning so that they act "faster" this round.

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I'm just going to take this opportunity to tell you the current house rule that my group uses for initiatives, which has its pros and cons, but the main pro is that its easy to do and keeps the table alert, as well as makes it easy to remember who goes next.

Basically, everyone rolls initiative as normal. The person who gets the highest initiative goes first. Then we follow the order of play clockwise around the table. Any summoned monsters, or hirelings take the initiative order of the player who summoned/hired them. All monsters go during the DM's turn. There is a big drawback to this in regards to tactics, but my group has been doing it and enjoying it long enough that I had forgotten that summoned creatures don't normally attach themselves to the summoner.

Answer 2

Go to the source! The PHB (p. 189) says:

1. Determine surprise.

2. Establish positions.

3. Roll initiative. Everyone involved in the combat encounter rolls initiative, determining the order of combatants’ turns.

4. Take turns.

5. Begin the next round.

and

When combat starts, every participant makes a Dexterity check to determine their place in the initiative order.

Why do you need to make any modifications to this for reinforcements? Just roll their initiative and have them act when their turn rolls around. It works perfectly well.

Some points to note:

1. Only the reinforcements can be surprised (unlikely if they can hear the combat but you never know - maybe the combat came to them); if they are they still roll initiative but cannot act on their first turn or react until after their first turn - this is the normal surprise rule.
2. The reinforcements can enter the combat hidden which, unless they are perceived, makes them untargetable until they stop being hidden. See What advantages does hiding have?

TL;DR

1. Yes.
2. No, but see above on surprise.
3. Yes. Yes.

Answer 3

As others have said, you can roll initiative and have the summoned creature act as normal initiative would dictate! Another way to play it is to group the summoner and the creature together, making them act one right after the other. It's your call really. But if you want RAW, you just roll initiative for it as soon as it is summoned and place it among the other combatants. This is true for summoned creatures as well as other kind of foes/allies that enter mid-combat.

For example, imagine the initiative order is:

• Adventurer 1: 21
• Adventurer 2: 18
• Summoner: 15
• Adventurer 3: 10

Then the summoner summons the creature. You roll initiative and you get a 19. You just introduce it into the initiative order like so:

• Adventurer 1: 21
• Monster: 19
• Adventurer 2: 18
• Summoner: 15
• Adventurer 3: 10