That's supposed to be easy...
House Cannith, Making -> Arcana + 1 Tool ('cos tool proficiency is mostly fluffy)
House Deneith, Sentinel -> Athletics (Mercenary) or Investigation (Sentinel Marshal) ('cos military, resistance, good with basic maneuvers like shove)
House Ghallanda, Hospitality -> Diplomacy + "Cooking Tool" (or something like it)
House Jorasco, Healing -> Medicine
House Kundarak, Warding -> Investigation (or Perception)
House Lyrandar, Storm -> "Water (or Air) Vehicles", plus ... Survival? Nature? I don't remember what rules over driving a ship, if it's a tool should be a bonus
House Medani, Detection -> Insight (or Investigation)
House Orien, Passage -> Arcana and "Land Vehicles"
House Phiarlan, Shadow -> Performance (or Acrobatics)
House Sivis, Scribing -> Insight (or History)
House Tharashk -> Survival (Perception ?)
House Thuranni, Shadow -> Stealth (or Deception)
House Vadalis, Handling -> Handle Animal
Edited due to great suggestions below
Investigation for House Kindarak didn’t feel right so I went back to the Eberron Campaign Setting book, re-read the dragonmarks section (p.63-67) and realised each Least Mark gave a bonus to a certain proficiency. If we convert that to advantage on the 5E version of each, we get…
Detection/Medani +2 Spot = advantage to Perception
Finding/Tharashk +2 Search = advantage to Investigation
Handling/Vadalis +2 Handle Animal = advantage to the same
Healing/Jorasco +2 Heal = advantage Medicine
Hospitality/Ghallanda +2 Diplomacy = advantage Persuasion
Making/Cannith +2 Craft = advantage … on Arcana checks relating to magic items and ability checks to make or repair items generally? Seems like Cannith should have a business edge for low magic campaigns and the Insight or Persuasion Guild Artisan background choices just seem to too removed from Cannith’s core “we’re the best at making stuff” powered feel (sure, except airships and elemental ships sure, and except traps too, but adding those bits goes beyond a House slogan
).
Passage/Orien +2 Survival = advantage … I can see how Survival would match guiding caravans through the wilderness, but what the Couriers Guild in urban settings not so much. I’d suggest a choice between Survival, or Acrobatics (somehow Orien couriers parkouring across towns feels like it fits better).
Scribing/Sivis +2 Decipher Script = advantage on … Intelligence checks relating to understanding written languages or codes? It sounds a bit vague but feels like it has a ncw chunk of adventuring potential.
Sentinel/Deneith +2 Sense Motive = advantage Insight (perfect for bodyguards trying to spot disguised would-be assassins and even for mercenary recruiters assessing candidates mettle)
Storm/Lyrandar +2 Balance = advantage … on ability checks relating to enduring, avoiding or taking advantage of weather effects? An instinctive feel for shifts in the wind helping balance checks on shifting ship decks, nascent abilities to summon sleet storms might call up the odd extra cool breeze in the desert (helping endure the elements), etc. Lyrandar scions might make great sailors but a feat is a hefty price tag, seems like it should be more broadly useful.
Warding/Kindarak +2 Search = advantage on … ability checks relating to finding, crafting or disabling traps?
It couldn’t wrap me head around it but Onslaught’s list mostly clicked, but the houses and the marks they’re built on seem more thematic than can be linked to just one proficiency sometimes.
Edit: i suggest Dragonmark feats give
- the Unearthed Arcana nominated spell like abilities, plus
- advantage to profiviency or ability checks as noted
i'd also go with one extra use per long rest of lesser mark powers each time your dragon mark leveled up to the next type, so by 9th level you can use a Least mark power 3 times per long rest and a lesser mark power twice per long rest.
