Traps & Hazards: The Open Portal

by Ameron (Derek Myers) on February 2, 2010

During the coming weeks we’ll be looking at traps and hazards in 4e D&D. I’m not going to waste time taking about pit traps or exploding doors, there’s plenty of that stuff in the DMG. What I’m going to focus on are traps and hazards that are much more elaborate and interesting.

These are things that make up a significant part of an encounter that is not the monsters. It’s usually going to take the form of a skill challenge or at the very least some skill checks. My intention is to provide examples of traps and hazards I’ve used in my campaign, describe how and why they were created, and give suggestions for using them in your campaign.

Hazard: The Open Portal


The PCs are travelling cross country. It’s winter and there’s already snow on the ground. Thus far the PCs have managed to keep ahead of an impending snow storm, but it’s catching up quickly. The snow’s begun falling and it’s going to get a lot worse very soon.


While the PCs seek adequate shelter they happen upon four elemental (cold) creatures. Three of the elemental beasts attack the PCs, while the last one remains in place and completely ignores the party. The creature that holds back does not enter the fight until the second round. It uses its actions on the first round to complete its task – opening a portal to its home plane.


The four monsters are all the same level as the PCs.

The Portal

  • Unless the PCs completely destroy the elemental creature performing the portal ritual before it acts in the first round, it succeeds in opening the portal.
  • The portal fills one square.
  • At the end of round two, and every round thereafter, an elemental minion of the PC’s level appears in a square adjacent to the portal (determine square randomly) and acts immediately.
  • A minion cannot appear in an occupied square.
  • Every round the portal is open, the number of minions that appear increases by one.
  • No more than five minions can travel through the portal per round, even if the portal is still open after the sixth round.
  • The number of minions travelling through the portal on any round cannot exceed the number of unoccupied adjacent squares.
  • Any creature that begins its turn on the portal or in an adjacent square of it takes 5 cold damage.
  • The portal remains open until the PCs close it.

Skill Challenge
Part 1 – Finding and Identifying the Portal


PC level +2 (Level 13)

Arcana (moderate DC, 18)

A PC within 1 square of the portal can identify it and determine its function.

A PC trained in Arcana can attempt to identify the portal from farther away. The PC must have a clear line of sight and must make a successful Perception check (moderate DC, 18). Add +1 to the DC for each square between the PC and the portal (because of the snow).

If the PC makes a successful Perception check, but fails the Arcana check they can keep trying to identify the portal until they are successful. They do not need to make another Perception check after achieving a success. The PC gains +1 bonus to the Arcana check after every failed attempt.

Identifying the portal counts as 1 success. A successful Perception check does not count as a success towards the overall the skill challenge.

Arcana (moderate DC, 18)

After the portal’s properties are identified, another successful Arcana check reveals that it is directly connected to the elemental’s life force. If the creature that opened the portal is destroyed, all checks to close the portal receive a +5 bonus.

Endurance (moderate DC, 18)

A PC trained in Endurance that makes a successful Endurance check receives a +1 bonus to save vs ongoing cold damage until the end of the encounter.

Part 2 – Closing the Portal

In order to close the portal, the PCs need to make 3 successful checks in a row. They do not have to be made by the same PC. If a PC fails a check the count resets and they must begin again.

Arcana (hard DC, 23)

The PC must be in an adjacent square to the portal in order to attempt to close it.

Arcana (moderate DC, 18, assist)

Use Arcana to assist a Thievery check made to close the portal.

Endurance (hard DC, 23)

As a minor action, a PC within 1 square of the portal can make an Endurance check to temporarily negate the ongoing cold damage emanating from the portal until the end of their next turn.

Insight (moderate DC, 18, assist)

A PC trained in Arcana realizes that arcane powers can also be used to destroy the portal. An arcane power with the fire keyword directed at the portal and dealing at least 1 point of fire damage counts as a success towards closing the portal.

Thievery (hard DC, 23)

The PC must be in an adjacent square to the portal in order to attempt to close it.

Thievery (moderate DC, 18, assist)

Use Thievery to assist an Arcana check made to close the portal.

DM’s Notes

The PCs in my group consist of two Bards, a Rogue and a Wizard. They are all very skills focused and their best skills are in the +15 to +18 range. So although the DCs for this skill challenge seem really high, I know that in my case these checks shouldn’t pose significant difficulty.

The real challenge depends on how many rounds pass before the PCs realize that the portal exists. At first a few extra minions may not pose real danger, but after a few more rounds they start to add up in a hurry. There’s a really good chance that the party will expend resources that target multiple creatures early in the combat, leaving them in a tough spot a few rounds later.

When I used this hazard I made sure that there were obstacles on the map that blocked sight, in this case a few trees. I had the monsters all appear from behind the trees in the first round. The portal was set up out of the PCs initial line of sight. As more and more minions started pouring out from the behind the trees a couple of the PCs decided to get a better look. They immediately realized that something was amiss.

In retrospect I think hiding the portal from initial view added an extra level of difficulty. The PCs were unaware of the additional hazard for at least three or four rounds. Since the portal already required the PCs to make a successful Perception check (with scaling difficulty) to pinpoint its location I should have just placed it in the open.

By adding this kind of hazard, the PCs needed to decide which posed the greater threat: the four initial elemental creatures or the portal that kept replenishing the minions. My players tried to kill all the monsters before worrying about closing the portal, but as more and more minions kept coming through they had to change tactics even though two of the initial monsters were well below bloodied.

Improving the Hazard

Creating this kind of add-on for your encounter is a lot less exact than throwing more monsters at your PCs. The success of this is tied directly to knowing your PCs. Had my players all been Fighters and none of them trained in Arcana, the portal would still be open.

In order for this series to be of the most benefit to the most people I encourage everyone to provide feedback. Let us know what you like or dislike about my traps and hazards. If you use them in your campaign, let us know how it worked out. I welcome your feedback and suggestions on how to improve this hazard.

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1 Neuroglyph February 2, 2010 at 10:46 am

Awesome hazard, and fairly adaptable to Characters at any tier, depending on how quickly you let the minions in.
.-= Neuroglyph´s last blog ..Review – Blackdirge’s Bargain Templates: Venomous by Blackdirge Publishing =-.

2 Mike D. February 2, 2010 at 11:59 pm

Excellent skill challenge. It also solves another problem I have with most of the encounters I run, which is: they are too static. Everyone just lines up in order to get combat advantage – I call it the Line Dancing Formation. I’m looking for more ways to incentive the PC’s (or the monsters, I suppose) to want to move around the battle field. Your constantly erupting Portal O’ Minions does exactly that.

3 Ameron February 3, 2010 at 10:04 am

Thanks. I’m glad you liked it. You can always tweak this to have more or less minions come through the portal. Or you can have a full fledged monster come through and then the portal needs to recharge before another can pass. That way the time interval becomes less predictable.

@Mike D.
I’ve run into the “Line Dancing Formation” a lot. In my current game I have the added complexity of two PCs who are predominantly ranged attackers. I struggle to keep things balanced and interesting for the melee and ranged combatants.

I’ve learned that bringing more monsters into combat during later rounds makes for much more interesting combat. It’s important to keep the encounter balanced from an XP point of view, but holding back a few until the third of fourth round and then having them come at the PCs for a different angle can really change combat dynamic.

4 skallawag February 3, 2010 at 1:18 pm

In a future adventure, I’m thinking of having the PCs walk down a hallway (in a dungeon) and then have the monsters push the wall down on them from inside of a room. Sometimes the PCs need to be caught off guard, plus a falling wall adds more to the dynamic (i.e. difficult terrain, damage from the wall, partial cover from all of the dust, combat advantage for the monsters, etc.).

5 Rook February 4, 2010 at 8:36 pm

I’ve got another name for it; the Minion Fountain. But whatever you call it, I like the idea of adding combatants into the fray over time. Nice encounter Ameron.

I also like Skallawag’s falling wall idea. I need to copy these down pronto.
.-= Rook´s last blog ..4e Magic Item Use: A Compromise =-.

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