Tribal and theme decks are always fun, but if you want to build a competitive deck it is often very difficult to have any type of tribe. This is because even with the traditional bonuses that can be given none of them were ever truly strong because any powerful card for one tribe would unbalance the rest and giving each tribe a truly powerful card would unbalance the game.
The solution to this is artifacts that do not check for specific creature types, but only matching creature types and the strongest of that style of card is “Coat of arms”.
Coat of arms is a rare artifact that has been in the core set of magic the gathering since seventh edition. It costs five colorless mana and its ability reads “Each creature gets +1/+1 for each other creature on the battlefield that shares at least one creature type with it. “
This means that if you have even two of the same creature type out they will get a bonus, and if you happen to play a group of them together it came be very powerful.
One of the points that is important to see in the wording of this card is that multiple matches on creature type do not add to multiple bonus. Having two goblin warriors in play will only give each of the goblins +1/+1 even though the match both keywords. On the other hand if you put a human warrior into play, all would get +2/+2 since they all have the warrior type. Then on the next turn if you put in a goblin shaman the goblin shamen would get +2/+2 because it matched the goblins, but the goblins would get +3/+3. Most importantly though, putting out two changelings that have every type would only give them +1/+1 rather than making them infinite size.
The weakness of this card is that it tends to make players over commit and become very susceptible to mass creature removal. Putting out 5 elves can seem very tempting , but if you opponent uses wrath of god to kill them all you are in trouble. The general solution to this is to use tokens. Consider conquerors’ pledge, a white sorcery from Zendikar that allows you to put 6 1/1 kor soldier tokens into play for three white and two colorless. With coat of arms already played this would mean that each of the creatures would be getting +5/+5. (it also has a kicker for 6 colorless that lets you put 12 tokens into play, meaning they would get +11/+11 but that is probably overkill.)
Coat of arms is a fantastic tribal card that in the right decks can be useful even in more competitive formats.