What is someone (or something) that causes phobia called?

Question

I'm looking for a word that describes someone who "could be a person or a object" that causes phobia.

For example, those with Anthropophobia could tell about people:

They're ________.

I recenlty this image (a meme, actually) where in Yu-Gi-Oh! series there is a character called Seto Kaiba which in a duel against Yami Yugi lost after Yami summons Exodia, The Forbidden One.

Exodiaphobia (fear to Exodia):

In this example, Exodia "or the one who summons him" can be called what?

Answer 1

Jim posted a good compound adjective in the comments: phobia-inducing.

Example of use:

Many phobia-inducing stimuli (such as blood, snakes, and heights) evoke fear in most people, and the decision as to when this becomes unreasonable or excessive depends to a degree on the investigators' judgment (and perhaps own fears)

("Specific Phobia", by Bavanisha Vythilingum and Dan J. Stein, in Clinical Manual of Anxiety Disorders, edited by Dan J. Stein, p. 46)

If you are feeling more adventurous, you could use a compound based on Greek roots like 0xFEE1DEAD's suggestion of "phobogenic" (adj.). Google Books provides a fair number of examples, all from pretty specialized contexts. A typical educated English speaker would not be expected to know this word, but might reasonably be expected to to be able to figure out the (approximate) meaning.

The expected equivalent noun to "phobogenic" would be "phobogen" (n.) (compare carcinogenic adj. and carcinogen n.), used as a count noun (e.g. "Spiders are a phobogen for me") but I haven't been able to find any examples of this hypothetical word being used at all. There is only one apparent true positive that I found via Google search ("In phobias, subjects are preoccupied by their fears only when confronted to the phobogen stimuli...") and here it seems to be used as an adjective, not as a noun (possibly as an awkward translation from an adjective in another language, like Spanish fobógeno/French phobogène, that would be more naturally translated as "phobogenic").

Answer 2

The word I would use is triggers (which is also available as a verb, trigger, and an adjective, triggerring). This is also the word used by many online articles that cover phobias. For example, Cleveland Clinic:

Agoraphobia is a separate diagnosis because it can have many potential triggers. A diagnosis of agoraphobia requires that you experience multiple types of triggers. The five all relate to being in public areas or spaces.

[...]

Behavioral symptoms are ways you change (or don’t change) your lifestyle, routine or habits. They include:

• Going out of your way to avoid triggers. Example: Avoiding regular medical or dental care because you’re afraid of needles, dentists, etc.
• Deliberately changing your life to avoid triggers. Example: Moving out of a coastal area because of a fear of hurricanes.
• Avoiding life changes — especially positive ones — so you don’t encounter triggers. Example: Avoiding buying a house or taking a job somewhere with a view of a river, lake or ocean because of a fear of water, swimming or drowning.

See also Mayo Clinic and WebMD.