Tier 1★ serves two primary functions: the first is to allow Tier 1 to be more inclusive, and the second is to give due recognition to the units that perform far above and beyond their peers. This is more of an extension of Tier 1 rather than a new tier in itself, to highlight units that overperform. Tier 1 units represent the most prominent and impactful units in the current metagame: they are among the strongest in the game and the best of their respective roles. Tier 1 units can perform exceptionally well in almost any team with little reliance on external support and have fantastic matchups against the majority of units.
Tier 2 units are among the strongest in the game, but are slightly less impactful than units in Tier 1. These units will have great matchups against many common Arena threats, making them some of the strongest choices for a core Arena team. With an ally or summoner support, and a synergistic team composition, many of these units can be just as powerful as those in Tier 1.
Tier 3 units are solid additions to most teams, with great matchups against common units. These heroes are more than capable in their favoured roles, but may have slightly weaker statlines than similar units in higher tiers. With that said, they can still perform their given role well with great success. Some of these heroes may be somewhat reliant on a support or sacred seal to perform as well as units in higher tiers.
Tier 4 units are reasonably strong in most teams, with good matchups against common units. Most of these heroes may be more niche or have less specialised stat distributions than units in the above tiers, but they can still perform their given role. They may be less effective that units in the above tiers however. Most of these heroes can be somewhat reliant on a support or sacred seal to perform as well as units in higher tiers.
Tier 5 units have decent match-ups against most units, but are often outclassed by units in higher tiers, or provide more niche use to a team. These heroes are by no means weak, but they are less consistent choices for arena teams, and are much more reliant on ally/summoner supports and sacred seals to perform effectively.
Tier 6 units have somewhat poor matchups against most units, and will require heavy investment from the rest of the team to be viable. These units are hard to recommend: many of these units will require a summoner support or ally support to repair awkward stat lines, whereas others are generally outclassed by units in the tiers above. With that said, Tier 6 units can be passable with enough investment.